2023 Subaru Forester 2.5L Oil Change — FB25D Boxer, 0W-20, Complete DIY Guide

I've owned Subarus for years and the FB25 boxer engine is one of my favorite engines to service. Flat, low-profile, everything is accessible — and the oil change procedure is straightforward once you know one quirk unique to the boxer: the horizontally-opposed cylinders drain oil sideways before it falls down, which means you need to let it drain longer than on a conventional engine. Leave the drain plug out for a full 10 minutes and you'll get a complete drain.

Here's everything you need to do an oil change on the 2023 Subaru Forester 2.5L.

Engine and Oil Specs

Item	Spec
Engine	2.5L FB25D naturally aspirated flat-four
Oil viscosity	0W-20 full synthetic
Oil capacity (with filter)	5.1 quarts
OEM oil filter	Subaru 15208AA15A
Aftermarket filters	FRAM PH9337, WIX 57356, K&N HP-1010, Mobil 1 M1-102A
Drain plug size	14mm hex
Drain plug torque	31 lb-ft (42 Nm)
Oil change interval	6,000 miles (conventional) / 7,500 miles (full synthetic)
Oil spec	ILSAC GF-6A, API SP

Subaru's official recommendation: Subaru Genuine Motor Oil 0W-20 (their OEM branded oil) is the spec. Any full synthetic meeting ILSAC GF-6A will work — Mobil 1, Pennzoil Platinum, Valvoline Full Synthetic. Do not use conventional oil on the FB25 boxer — the design has tight oil passages and the low-friction coating on the cylinder walls benefits from full synthetic.

Drain plug torque note: 31 lb-ft is higher than many comparable four-cylinders. The Forester steel oil pan requires it — under-torque leads to slow seep at the crush washer. Use a torque wrench.

Why the FB25 Drains Differently

The horizontally-opposed (boxer) engine lays the cylinders flat. Oil in the cylinder heads, lifters, and upper oil passages drains sideways along the head before it falls through the drain passages into the sump. On a conventional inline-four, gravity pulls everything straight down quickly. On a boxer, the drain is more gradual.

Practical impact: Let the oil drain for a minimum of 10 full minutes after removing the drain plug. 5 minutes is not enough on the FB25. You'll leave 0.3–0.5 quarts of oil behind in the head passages if you rush.

Tools and Parts List

• 6 quarts of 0W-20 full synthetic (buy 6 for extra; engine takes 5.1 with filter)
• Subaru 15208AA15A filter or equivalent
• 14mm socket or box wrench
• Torque wrench (25–45 lb-ft range)
• 6–7 quart drain pan (boxer engines drain with slightly more spread)
• Funnel
• Rags or shop towels
• Drain plug crush washer: Subaru 803916010 (replace every oil change)
• Floor jack and stands (or Subaru-height ramps)

Crush washer note: Subaru recommends replacing the aluminum crush washer every oil change. It deforms to seal the drain plug face. Reusing a deformed washer can cause slow seepage, especially at Subaru's higher 31 lb-ft torque spec. Buy a few at a time — Subaru 803916010 is about $1.50 each.

Step-by-Step Oil Change

1. Warm the Oil

Run the engine 3–5 minutes. Shut off, wait 5 minutes. On the FB25 boxer, warm oil is especially important — cold thick oil clings to the horizontal cylinder walls and drains slowly even with a long drain time.

2. Access the Underside

The 2023 Forester has a plastic engine undershield. It's attached with 10mm bolts and push-pin clips — typically 4 bolts across the front edge and two pins in the middle. Remove the shield completely and set it aside. You need full access to both the drain plug and the oil filter.

3. Drain the Oil

Position the drain pan under the oil pan drain plug — on the FB25, it's at the rear of the oil pan, slightly to the right of center (passenger side). Remove the 14mm drain plug counterclockwise.

Set a timer for 10 minutes. Walk away. The boxer drainage physics require patience.

While draining, remove the oil filler cap on the top of the engine (driver's side, above the left cylinder bank). This equalizes pressure and speeds drainage.

4. Replace the Oil Filter

The Subaru 15208AA15A is a canister spin-on filter located on the passenger side of the engine, toward the front. On the 2023 Forester, it's accessible from the driver's side of the engine bay (reach over the engine) or from below once the undershield is off.

Unscrew counterclockwise. Have a rag positioned — the filter will dribble oil. Confirm the old rubber O-ring comes off with the filter.

Wipe the seating surface clean. Apply a thin coat of fresh 0W-20 to the new filter's O-ring. Thread on hand-tight, then ¾ turn past hand-tight.

5. Reinstall the Drain Plug

Install the new crush washer (Subaru 803916010) onto the drain plug. Thread in by hand to avoid cross-threading. Torque to 31 lb-ft (42 Nm).

Do not skip the new crush washer. The old deformed washer will not re-seal reliably at 31 lb-ft.

6. Fill with 0W-20

Pour 4.5 quarts first. Replace the filler cap. Start the engine — the oil pressure light should go off within 5 seconds. Idle for 60 seconds. Shut off, wait 3 minutes (slightly longer than typical engines — allow oil to settle back into the flat engine's sump), then check the dipstick.

The FB25 takes 5.1 quarts with a filter change. Add incrementally to reach the FULL mark. Overfilling a boxer causes oil to be forced into the cylinder heads more aggressively during lateral load — don't exceed FULL.

7. Inspect for Leaks

Run the engine 2 minutes, then check the drain plug and filter area from below for drips. Shut off and do a final dipstick check.

8. Reinstall the Undershield

Reattach with all bolts and pins. Don't leave the undershield off — it protects the oil pan from road debris and keeps the drain plug accessible on a flat surface.

9. Reset the Maintenance Reminder

On the 2023 Forester:

• Turn ignition to ON (engine off)
• Press and hold the SELECT and RESET buttons on the instrument cluster simultaneously for 5+ seconds
• The maintenance minder will reset

Or, on models with the Starlink infotainment: Settings > Vehicle > Maintenance > Reset Oil Life.

Cost Comparison

	DIY	Subaru Dealer
0W-20 full synthetic (6 qt)	~$35–$48	—
Subaru 15208AA15A filter	~$8–$14	—
Crush washer x2	~$3	—
Labor	$0	$60–$90
Total	~$46–$65	$110–$165

At 7,500-mile intervals with full synthetic, that's one to two changes per year. You're saving $65–$100 each time.

Common Mistakes

1. Not waiting 10 minutes for the oil to drain. The boxer engine is the main reason this one trips people up. Set a timer.

2. Reusing the crush washer. Subaru rates this washer as single-use. Reuse + 31 lb-ft torque = likely slow weep within a few thousand miles.

3. Overfilling past FULL. The FB25 is especially sensitive to overfill. The horizontally-opposed design means excess oil contacts the rotating assembly differently than in a vertical engine. Check the dipstick carefully.

4. Using 5W-30 from memory. Earlier Forester models (pre-2017 on the FA20 engine) used 5W-30. The FB25D in the 2023 Forester is specced for 0W-20. Check the filler cap.

5. Installing the filter without oiling the O-ring. On the Subaru filter, a dry O-ring won't seat correctly and can spin off on first startup under oil pressure.

FB25 Long-Term Reliability

The FB25D is a robust naturally aspirated engine. With fresh 0W-20 at the correct interval, it routinely sees 200,000+ miles without major issues. The head gaskets on the FB25D (the D revision) are significantly improved from earlier EJ-series and early FB-series engines — not an expected failure point. The cam carrier o-rings are worth inspecting at 100,000+ miles as part of a valve cover gasket job.

For more Subaru DIY content, see the 2015 Subaru Legacy timing belt vs chain guide and the how to do an oil change at home without a mess guide if this is your first DIY oil change.

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