Parts for your 2018 Subaru Legacy-Sump plug washers

2018 Subaru Legacy sump-plug-washers — what they do and when to replace them

Yes, the 2018 Subaru Legacy uses a sump-plug-washer (crush washer) on the engine oil drain plug. This is confirmed in Subaru’s technical literature: the Legacy/Outback (2015–2019) Service Manual for FB-series engines specifies fitting “a new drain plug gasket” and tightening the plug to the listed torque, the 2018 Legacy Owner’s Manual instructs replacement of the drain plug gasket during oil changes, and the Subaru parts catalogue lists a 16 mm drain plug gasket (often referenced as p/n 803916010) applicable to the 2018 Legacy. These sources collectively show the washer is a required service item.

On the 2018 Subaru Legacy, the sump-plug-washer is a soft, single-use crush washer that sits between the sump (oil pan) and the drain plug. Its job is simple but critical: when the plug is tightened, the washer deforms slightly to take up microscopic gaps, creating a reliable seal that keeps fresh oil in and road grime out. That tiny ring prevents weeping, slow drips on the driveway, and that annoying hot-oil whiff under the bonnet after a long run.

Because the washer is designed to compress once, it should be replaced at every oil change. Re-using a crushed washer can lead to under-sealing (oil seep) or over-tightening to “chase” a seal, which risks damaging threads or the sump face. The best practice for the 2018 Legacy is straightforward:

• Use a new, correct-size 16 mm aluminium crush washer that matches Subaru specs.
• Clean the sump face and drain plug, fit the washer in the correct orientation if it’s chamfered, and tighten to the specified torque (commonly 33 N·m for the FB25 engine, always confirm for your engine variant).
• Don’t add thread tape or sealants — the crush washer provides the seal when torqued properly.

Signs a washer needs attention include a light oil halo around the plug, the odd drip after parking, or oil mist on the undertray. If any of that shows up after a service, it’s worth replacing the washer and re-torquing the plug to spec. For Aussie and Kiwi owners, it’s handy to keep a couple of genuine-style washers in the glovebox — they’re inexpensive and save mucking around on service day.

Whether the car is doing daily commuting or big open-road kilometres, a fresh crush washer at each oil change is cheap insurance for a tidy, leak-free sump and long engine life.

Popular questions about 2018 Subaru Legacy sump-plug-washers

How often should the sump-plug-washer be replaced on a 2018 Legacy?
It should be replaced at every engine oil change. The washer is a single-use crush design, so once it’s compressed, it won’t seal as reliably the second time. If the car follows typical AU/NZ service intervals, that’s generally every 10,000–15,000 km or 12 months, depending on conditions and the service schedule.

What size and type washer does the 2018 Legacy use?
The 2018 Legacy uses a 16 mm aluminium crush washer designed for Subaru drain plugs. Genuine-style washers match the sump face and plug head profile, making it easier to achieve a proper seal at the correct torque. If in doubt, confirm by VIN or with the engine variant.

Can the washer be reused if it “looks fine”?
Not recommended. Even if it looks tidy, the metal has already deformed. Re-using can cause weeping or force over-tightening. A new washer plus correct torque is the reliable way to keep the sump dry and threads healthy.