GetCoattail Research — The Filing Tape #61. Marathon Petroleum is the largest U.S. refiner, and its shares have been carried by a diesel squeeze: a 55.1% return in the third quarter of 2026, the best of the big refiners, a place on the Wall Street Journal's October 12 stocks-to-watch recap, and a Barclays target raised the same day from $321 to $488. The filings describe a register moving the other way at the top: the tracked superinvestor cohort cut its position 91.8% in the second quarter — the entire decline in a single holder — while the full 13F market added 35,085,379 shares, insiders recorded no purchase at all in a year, and Congress recorded sales only.
Start with the narrative, because it is a strong one. Refiners were the energy sector's leaders in the third quarter of 2026, and Marathon Petroleum led the refiners: a 55.1% quarterly return, ahead of Phillips 66 at 51.8% and Valero at 49.4%, in a quarter when energy gained 16.5% against the S&P 500's 2.0%, by Forbes columnist Robert Rapier's accounting. The driver was the gap between crude and fuel prices. U.S. diesel set records in September — $6.52 a gallon on September 22 by one market report, with the Energy Information Administration's weekly national average peaking at $6.529 in late September — as damaged and restricted refining capacity abroad met inventories that were already thin. On October 2 the G7 agreed to release up to 100 million barrels of emergency oil and diesel stocks over four months in response. The analyst register has been chasing the move: on October 12, Barclays raised its Marathon Petroleum target from $321 to $488 and kept an overweight rating — a 6.48% implied upside from the prior close, on the same day the Wall Street Journal's stocks-to-watch recap listed Marathon Petroleum and Valero among the day's names as investors weighed U.S. moves to boost diesel supplies. Mizuho had raised its target from $304 to $457 on October 8. Counts published with the Barclays note put the consensus at Moderate Buy with an average target of $395.38, from 1 strong buy, 11 buys and 5 holds. Underneath sits the most recent reported quarter, published August 4: earnings of $17.73 a share against a $14.27 consensus, on revenue up 53.5% year over year to $51.99 billion. Our own price file, which ends October 2, closes MPC at $422.33 — 0.6% below the 252-session closing high of $424.89 set on September 18, 2026, up 159.7% year to date, and up 65.2% since the June 30 snapshot the 13F lenses below are taken from. The next scheduled tests are the third-quarter print on November 3, 2026, and the third-quarter 13F season closing in mid-November.
Now the filings. For MPC — the sixty-first Filing Tape audit — the tape shows a three-name tracked cohort that nearly exited in a single quarter, a broad register that added 35.1 million shares across 76 more filers, a 5% register of passive complexes re-papering a Vanguard stake, an insider record with no purchase in a year, a congressional record of sales only, and a borrow sitting at a fifth of its 2021 peak.
Lens 1: the superinvestors — three holders, and one of them cut 91.8%
We track 83 large, mostly long-only managers whose 13F portfolios we collect directly from SEC EDGAR. As of June 30, 2026, 3 held MPC long totalling 231,255 shares worth $59,124,965, with no option rows in the register. The direction count on the collection's own labels, across the long rows, is one decrease and two unchanged. The cohort total moved from 2,808,216 long shares at March 31 to 231,255 at June 30, a change of -91.8% — a difference of -2,576,961 shares. The collection's exit list names no MPC departures, because the register did not shrink by heads. It shrank by size, in one row.
That row is Bill Nygren - Oakmark Funds, with 229,011 shares worth $58,551,242, a 0.08% portfolio weight — 99.0% of the tracked cohort's MPC shares — flagged decreased by 91.8%, and the pipeline's March file carries the position at 2,805,972 shares, so the label and the continuity agree: the entire cohort decline of 2,576,961 shares is the Oakmark decline of 2,576,961 shares. The other two holders did not move at all. Mairs & Power Funds holds 1,390 shares, unchanged, and David Katz - Matrix Asset Advisors holds 854 shares, unchanged; the March pipeline rows for both are unnamed in the file and match on share counts, and both agree with their unchanged labels. The cohort's implied price, $255.67 per share, sits exactly on the June 30 close of $255.67. This register's second quarter, in full: one large holder reduced a position by more than nine-tenths, two small holders stood still, and nobody new arrived — in a quarter when the price rose from $244.18 to $255.67, a change of +4.7%.
Lens 2: institutions — 76 more filers, 35.1 million more shares
Zoom out to the whole 13F market and the register moved the opposite way from the tracked cohort on every measure. At June 30, 2026, 1,933 institutions reported positions, up 76 from 1,857 at March 31 (+4.09%), and 1,929 of them reported long positions. Reported shares rose 19.35%, from 181,294,688 to 216,380,067 shares — 35,085,379 more shares than the prior quarter. Reported aggregate value rose 24.04% to $54,152,516,638; the aggregate includes option rows, so we headline filer and share counts and label dollars "reported" (see methodology). The implied value per share, $250.27, sits 2.1% below the June 30 close of $255.67 — a gap published as found. Where the tracked cohort's register is three names and a near-exit, the broad register is nearly two thousand filers who, in aggregate, ended the quarter holding a fifth more shares than they started it with.
Lens 3: the 5% register — six passive filings, one re-papering
Our full parse holds exactly 6 SC 13D/G filings for MPC — two SC 13G and four SC 13G/A, and no SC 13D-family filing at all — four reporting persons, the most recent filed April 30, 2026. The register's maximum is The Vanguard Group at 38,865,857 shares (12.65%) as of June 30, 2025, filed July 29, 2025, up from 34,941,144 shares (11.21%) as of March 31, 2025. The same reporter's next row is 0 shares (0.0%) as of March 13, 2026, filed March 27, 2026 — and the stake then reappears under two sibling entities at the next date: Vanguard Capital Management prints 22,147,567 shares (7.52%) as of March 31, 2026, filed April 30, 2026, and Vanguard Portfolio Management prints 15,765,266 shares (5.35%) as of March 31, 2026, filed April 29, 2026. The fourth name is BlackRock, Inc., at 21,060,653 shares (6.8%) as of March 31, 2025, filed April 23, 2025. That is the whole 5% register in our parsed coverage: two index complexes, one of them moving its reported stake across reporting entities, and no activist paper of any kind.
Lens 4: insiders — no purchase in a year
The parsed Form 4 record holds 87 filings through October 2, 2026. Across the whole record, open-market purchases total 2 transactions, totalling 3,000 shares and $403,140.00, by two buyers, one of them recorded as an officer purchase of $269,440.00. Sales total 19 transactions, totalling 63,071 shares and $16,938,866.99, by six sellers, and 18 of the 19 are recorded as officer sales. The recent windows are emptier on the buy side than the full record: the last year holds 37 filings with 0 purchases against 17 sales totalling 44,800 shares and $13,871,989.99 by five sellers, every one of them an officer sale; the last 90 days hold 6 filings with 0 purchases and 8 sales totalling 27,702 shares and $9,833,225.90 by four sellers, again all officer sales. Our site's insider aggregate, compiled as of October 13, matches the full parse's 90-day window exactly, and carries no last-buy date for this name. We assign no motives to any of it; the record itself — purchases that exist in the older record and have not occurred at all in the most recent year, against officer selling in every window — is the fact.
Lens 5: Congress — no purchases, three sales, House only
The congressional record we maintain is one-directional for this name, in the opposite direction from the last audit in this series. The combined record (as of October 3, 2026) carries 0 recorded purchases and 3 recorded sales, with no recorded purchase date; the aggregate's member count for this ticker reads zero, and we report the transaction counts as filed rather than reconstructing who filed them. The House aggregate (as of October 3, 2026) carries the same 0 purchases and the same 3 sales — the combined record is the House record. The Senate aggregate (as of October 1, 2026) carries no MPC row at all. No recorded purchase in either chamber's record as this site maintains them.
Lens 6: short interest — 7,715,203 shares, a fifth of the 2021 peak
FINRA's files carry the borrow across 137 settlements, from January 15, 2021. As of September 15, 2026 — the most recent settlement — 7,715,203 MPC shares were sold short, with days-to-cover of 3.22. That reading rose 1.5% from the prior settlement's 7,601,383 shares on August 31, 2026, completing two consecutive increases from 6,521,760 on August 14, after 6,786,536 on July 31 and 6,412,255 on July 15. Against September 15, 2025 the latest reading is down 1.9% from 7,861,133 shares. Both of those statements are small next to the larger one: the series maximum is 36,223,577 shares on March 15, 2021, a fact we verified by taking the maximum across the whole series rather than reading it off the latest row, and the latest reading sits 78.7% below that peak — the borrow today is roughly a fifth of what it was in early 2021, when the series' first settlement, on January 15, 2021, printed 35,060,589 shares. Against the fundamentals file's 280,824,763 shares outstanding, the latest reading is 2.75% of the share count. The June 30, 2026 settlement — the date the 13F lenses snapshot — printed 7,290,864 shares; the latest reading sits 5.8% above it. The series minimum is 5,585,954 shares on April 30, 2026; the latest reading sits 38.1% above that low, so the borrow has been rebuilding, slowly, from its spring floor.
The borrow is neither crowded nor gone. It is a 2021-sized position reduced by four-fifths, rebuilding by tenths.
The gap, stated plainly
The narrative describes a refiner at the centre of a supply shock being re-rated in real time: the best third-quarter return of the big refiners at 55.1%, record diesel prices in September, a Barclays target raised to $488 on October 12 against a Moderate Buy consensus averaging $395.38, an August earnings print that beat by $3.46 a share, and a stock 0.6% below its 252-session closing high and up 159.7% year to date on our price file. The filings describe the positioning that preceded the loudest part of it. The tracked superinvestors held 231,255 shares at June 30 across three holders, having cut 91.8% in a quarter when the price rose 4.7% — with one holder, Oakmark, carrying 99.0% of what remains after reducing its own position from 2,805,972 shares to 229,011. The full market moved the other way: 216,380,067 reported shares, up 35,085,379, across 1,933 filers — 76 more than the prior quarter. The 5% register printed 6 filings, all passive: a Vanguard stake whose maximum print is 12.65%, re-papered in 2026 from The Vanguard Group into two sibling entities at 7.52% and 5.35%, alongside a BlackRock row at 6.8%. Insiders, across 87 parsed filings, recorded 2 purchases in the whole record and none at all in the last year, against 19 sales of which 18 were officer sales — including 8 officer sales in the last 90 days alone. Congress records 3 sales and no purchases, all in the House record. And the borrow stands at 7,715,203 shares — down 1.9% in a year and 78.7% below its March 2021 peak, yet up 38.1% from its April 2026 low and rising for two settlements. One plausible reading is that the registers are describing a name the broad market accumulated and the concentrated cohort declined to chase: the full register added a fifth more shares while the tracked cohort's one material holder cut by nine-tenths, the insider tape sold without buying for a full year, the congressional tape sold without buying at all, and the short side — the register that would profit from the squeeze ending — sits at a fifth of its former size and has only just begun to rebuild from its floor — all of the 13F part of it snapshot at June 30, when the stock closed at $255.67, before the third-quarter run to $422.33 that the current story rests on. The record supports each clause separately; it does not say which register is right, and it cannot — the tracked cohort's whole remaining position is a June 30 snapshot of 231,255 shares, taken before the quarter that made the narrative.
What would change the picture
Nothing above is a recommendation. It is a record. These are the filings that would change the description:
- 1. The 2026Q3 13Fs (filing window closing mid-November 2026). The tracked cohort held 231,255 shares at June 30 across three holders, after a -91.8% move driven entirely by Oakmark's cut to 229,011 shares. Whether Oakmark rebuilds toward its March position of 2,805,972 shares, holds, or exits the register entirely — and whether any fourth tracked name arrives — are the first things the next register will answer.
- 2. A first insider purchase in over a year, at any size. The parsed record holds 2 purchases in total and none in the last year or the last 90 days, against 19 sales. A single open-market purchase would end a buy-side silence that currently covers the most recent twelve months of the record.
- 3. Whether the borrow keeps rebuilding from its floor, or fades again. Short interest stands at 7,715,203 shares, 38.1% above the April 30, 2026 low of 5,585,954 and 78.7% below the March 15, 2021 peak of 36,223,577, having risen at each of the last two settlements. A reading back below the July 15 print of 6,412,255 would resume the fade; a sustained move above the June 30 settlement's 7,290,864 is already in place and would need to hold.
- 4. A first congressional purchase, or a Senate row. The combined congressional record carries 0 purchases and 3 sales, all in the House record, and the Senate carries no row. A first recorded purchase in either chamber would change this lens from sales-only into a two-sided one.
- 5. A first SC 13D, or a new 5% name. The 5% register in our parsed coverage holds six passive filings from two complexes and no SC 13D-family filing. An original 13D from any filer, or a 13G from outside the Vanguard and BlackRock complexes, would give this lens its first non-passive content.
Until one of those prints, the record describes a stock held by three of our 83 tracked superinvestors — 231,255 shares, cut 91.8% in the quarter its price rose 4.7%, with one holder carrying 99.0% of the stake — whose full market added 35,085,379 reported shares across 76 more filers, whose insiders have recorded no purchase in a year against 19 sales, whose congressional register is 3 sales and no purchases, and whose borrow has fallen 78.7% from its 2021 peak while rebuilding 38.1% from its April low — all of it snapshot at June 30, before the third-quarter run and the target raises the current story rests on.
Related reading: Ten of 83: One Holder Is 58.9% of the Superinvestors' Salesforce Stake. No Officer Has Ever Bought a Share. — Filing Tape #60; Three of 83: The Superinvestors Cut Lululemon 86.0% in Q2. Short Interest Just Hit a Record. — Filing Tape #59; Zero of 83: The Superinvestors Aren't in IREN — Filing Tape #1. MPC's own page: Marathon Petroleum Corp (MPC) on GetCoattail.
Methodology and data notes
- Superinvestor lens: 83 managers collected directly from SEC EDGAR Form 13F (managers_13f collection), positions as of June 30, 2026; MPC identified by CUSIP 56585A102. Three distinct managers hold long positions totalling 231,255 shares; there are no option rows for this name in the collection. The direction count on the collection's labels, across the long rows, is one decreased and two unchanged; the collection's exit list contains no MPC rows. Quarter-to-quarter cohort totals (2,808,216 long shares at March 31; 231,255 long shares at June 30; change -91.8%) are from the site pipeline's holdings files on the same CUSIP basis in both quarters, and the pipeline's second-quarter totals match the EDGAR collection exactly on both shares and value (differences of 0 shares and $0). Per-holder quarter changes are matched by filing continuity across the two pipeline files: Oakmark Funds 2,805,972 → 229,011 = -91.8%, agreeing with the EDGAR collection's own percentage label; the two unnamed March rows are matched on share counts to Mairs & Power Funds (1,390 → 1,390 = 0.0%) and Matrix Asset Advisors (854 → 854 = 0.0%), both agreeing with their UNCHANGED labels. Oakmark's share of the cohort is 99.0% = 229,011 / 231,255, and the cohort's entire share decline (-2,576,961) equals the Oakmark row's decline (2,805,972 − 229,011 = 2,576,961).
- Full-market lens: SEC Form 13F Data Sets full-market aggregation. The primary table is the full13f collection's by-ticker file (holders = distinct filers reporting the CUSIP; 1,929 reported long positions out of 1,933 filers). MPC reports under a single CUSIP spelling, 56585A102, and the backfill fixtures' by-CUSIP file carries the same row — the cross-check matches on every field (holders, shares, reported value, prior holders and prior reported value). The by-ticker file's entered/exited fields (1,512 and 1,436, a net of +76 that matches the change in holder count) are gross flows, so this audit headlines the net change. Reported-value caution: the aggregate value ($54,152,516,638 over 216,380,067 shares) implies $250.27 per share against a June 30, 2026 close of $255.67 in our price file, a gap of -2.1% — printed here as found. The aggregate sums the VALUE of every information-table row — including put and call option rows — while the share count covers long common-stock rows only, so we headline filer and share counts and label dollar values "reported."
- Insider lens: full-parse Form 4 aggregation (agg_by_ticker), record as of October 2, 2026 (87 filings). Sale and purchase counts are transaction-code counts (S and P) in non-derivative rows; grants and exercises are not counted as purchases, and one filing can contain many transaction rows. Purchase transactions total 2 across the parsed record (3,000 shares, $403,140.00, two buyers, one of them recorded as an officer purchase, $269,440.00); the last year holds 0 and the 90-day window holds 0. Sale transactions total 19 across the record (63,071 shares, $16,938,866.99, six sellers, 18 of them recorded as officer sales); the last year holds 17 (44,800 shares, $13,871,989.99, five sellers, all 17 officer sales) and the 90-day window holds 8 (27,702 shares, $9,833,225.90, four sellers, all 8 officer sales). The site's insider aggregate (compiled as of October 13, 2026) matches the full parse's 90-day window exactly (0 purchases; 8 sales, 4 sellers, $9,833,225.90; no most-recent purchase date carried for this name), so this audit prints one 90-day count rather than two.
- 13D/G lens: full-parse SC 13D/G parsed filings (6 filings matching issuer CIK 1510295 on CUSIP 56585A102, 6 reporting-person rows across 4 names: two SC 13G filings and four SC 13G/A amendments, no SC 13D-family filings), filings through April 30, 2026. The site aggregation's by-ticker file carries the same totals (6 filings, 4 reporters, latest filing April 30, 2026, maximum 12.65%, total reported 132,780,487 shares), so the aggregate and the parse agree here; the aggregation's total sums reporting-person rows across different dates and entities, so it is not used as a stake figure in the article. The Vanguard Group's rows (34,941,144 shares at 11.21% as of March 31, 2025, filed April 30, 2025, SC 13G/A; 38,865,857 shares at 12.65% as of June 30, 2025, filed July 29, 2025, SC 13G/A; 0 shares at 0.0% as of March 13, 2026, filed March 27, 2026, SC 13G/A), Vanguard Capital Management's row (22,147,567 shares at 7.52% as of March 31, 2026, filed April 30, 2026, SC 13G), Vanguard Portfolio Management's row (15,765,266 shares at 5.35% as of March 31, 2026, filed April 29, 2026, SC 13G) and BlackRock, Inc.'s row (21,060,653 shares at 6.8% as of March 31, 2025, filed April 23, 2025, SC 13G/A) are reported as filed, under the reporting-person names exactly as they appear. No reason for any change is asserted — the filings themselves are the record. The parsed coverage for this issuer holds these 6 filings; a 5% holder that has not filed in the covered window would not appear here.
- Congress lens: Senate PTR archive and House PTR archive as maintained on this site. The combined record (as of October 3, 2026) carries 0 recorded purchases and 3 recorded sales, no most-recent purchase date, and an aggregate member count of zero for this ticker; the House aggregate (as of October 3, 2026) carries the identical counts, so the chamber sum checks: House (0 purchases, 3 sales) + Senate (no row) = combined. The Senate aggregate (as of October 1, 2026) carries no MPC row at all. Transaction counts are reported as the aggregates carry them; this audit does not reconstruct individual filers from the aggregate.
- Short interest lens: FINRA Equity Short Interest files (biweekly settlement dates), collected directly from FINRA's public distribution. The published series covers 137 settlements from January 15, 2021 (35,060,589 shares) to September 15, 2026. The latest reading is 7,715,203 shares on September 15, 2026, up 1.5% from the prior settlement (7,601,383 on August 31, 2026) and the second consecutive increase after 6,521,760 on August 14, 2026, which followed 6,786,536 on July 31 and 6,412,255 on July 15. The series maximum is 36,223,577 shares on March 15, 2021 (verified by a maximum query over the series, not read off the latest row), so the latest reading sits 78.7% below the peak; against September 15, 2025 (7,861,133 shares) it is down 1.9%. The series minimum is 5,585,954 shares on April 30, 2026, so the latest reading sits 38.1% above the low. The June 30, 2026 settlement printed 7,290,864 shares, so the latest reading sits 5.8% above the 13F snapshot date's borrow. FINRA publishes each settlement about two weeks after the fact, so the fiscal third-quarter earnings period expected later in 2026 is not yet in this series. The share-count ratio (2.75%) uses the fundamentals file's shares-outstanding figure below.
- Fundamentals note: our fundamentals collection carries MPC's revenue of $132,699,000,000 labelled fiscal 2025, its net income of $4,047,000,000 labelled fiscal 2025, and its assets of $83,955,000,000 labelled fiscal 2025. The file is marked ok for this name, with no missing fields: shares outstanding of 280,824,763 (as of July 30, 2026 in the file, sourced in the file to the issuer's reported shares-outstanding figure) and a file-computed market value of $121,417,390,417.04. The file's price for the name is $432.36 as of October 6, 2026; our daily-close file ends October 2, 2026 at $422.33, and we keep the two files' figures separate rather than blending them.
- Prices: our own daily-close file for MPC through October 2, 2026 ($422.33 close, +159.7% year to date on that file; 252-session closing high $424.89 on September 18, 2026; 252-session closing low $162.63 on December 31, 2025; closes of $244.18 at March 31, 2026 and $255.67 at June 30, 2026, a second-quarter change of +4.7%; change from June 30 to October 2 of +65.2% ((422.33 / 255.67) − 1); change from the 252-session low of +159.7% ((422.33 / 162.63) − 1), and the drawdown from the 252-session closing high is -0.6%. The series in our price file begins June 1, 2021 at $62.50.
- Market context sources: Wall Street Journal, Stocks to Watch Recap (October 12, 2026; Valero and Marathon Petroleum shares edged up as investors weighed U.S. moves to boost diesel supplies — checked October 13, 2026); third-quarter refiner returns (Marathon Petroleum +55.1%, Phillips 66 +51.8%, Valero +49.4%; energy sector +16.5% vs S&P 500 +2.0% — Forbes columnist Robert Rapier's analysis as reported by KarmActive, checked October 13, 2026); diesel prices (U.S. diesel at $6.52 a gallon on September 22, 2026 — Kalkine market report, checked October 13, 2026; EIA weekly national average on-highway diesel peaking at $6.529 a gallon in late September 2026 — Morningstar/PR Newswire, October 6, 2026); the G7 release (agreement of October 2, 2026 to release up to 100 million barrels of emergency oil and diesel stocks over four months — Morningstar/PR Newswire, October 6, 2026); the Barclays note (October 12, 2026; target raised from $321 to $488, overweight, 6.48% implied upside from the prior close; consensus Moderate Buy with an average target of $395.38, from 1 strong buy, 11 buys and 5 holds — Zolmax reporting the Barclays note via Benzinga and MarketBeat counts, October 12, 2026); the Mizuho note (October 8, 2026; target raised from $304 to $457, neutral — as reported by Zolmax and Daily Political, October 2026); the most recent reported quarter (published August 4, 2026; earnings of $17.73 a share against a $14.27 consensus; revenue up 53.5% year over year to $51.99 billion — MarketBeat, checked October 13, 2026); the next earnings date (November 3, 2026 — Zacks, October 5, 2026). Figures we could not confirm against two sources were left out.
- MPC is in the GetCoattail stock-page universe: Marathon Petroleum Corp (MPC) stock page. Series data: mpc_lens_data.csv · mpc_short_interest_series.csv. Earlier audits: Salesforce · Lululemon · IREN.
Past patterns are not a promise of future results. For education only — not investment advice.
This article describes public filings as of the dates stated. It is not investment advice and makes no recommendation. Filings lag reality: 13F positions are as of June 30, 2026, and later trading is not yet public.
Revision history
- v1.0 — October 13, 2026: first publication.