Salt-water corrosion. Water-injected cylinders. Extended-drain reservoir boats.
Potholes Reservoir, Priest Rapids, and the Columbia basin put marine engines through duty cycles car motor oil was never built for — post-fog water ingestion, off-season layup in a salt-mineral environment, and drain intervals that are measured in hours and seasons rather than miles. AMSOIL's marine and marine-diesel lines are formulated around those exact conditions, and every order runs through the dealer referral so Preferred Customer pricing carries through to checkout.
Already running AMSOIL? Become a Preferred Customer
Three lines for inboard, outboard, and marine diesel.
Pick the line that matches your platform — each one is tuned to the duty cycle and the certification spec that line actually carries. Click through to amsoil.com to confirm fitment; Preferred Customer pricing rides along on the dealer referral tag so the price at checkout is the dealer rate.
Marine 10W-30 / 25W-40
AMSOIL’s 100% synthetic marine engine oil — formulated for 4-stroke inboard and outboard power. NMMA FC-W certified, anti-foam and demulsibility tuned so post-fog-cycle water separates out of the sump instead of emulsifying into the additive package, and the rust inhibitor keeps bearings and cam lobes passivated through off-season layup on the reservoir.
Best for: inboards, 4-stroke outboards, sterndrive, I/O platforms.
Shop on AMSOILHP Marine 2-Stroke
AMSOIL’s HP Marine synthetic 2-stroke oil — for oil-injected or pre-mix outboards where ash and plug fouling are the failure modes. The synthetic base burns cleanly and resists the varnish deposits conventional TC-W3 oils leave on piston crowns and exhaust ports, and the package is qualified to NMMA TC-W3 for the platforms that still call for it.
Best for: 2-stroke outboards, oil-injected or pre-mix, older Tohatsu / Johnson / Evinrude.
Shop on AMSOILHeavy-Duty Marine Diesel 15W-40
AMSOIL Signature Series Max-Duty Synthetic Diesel Oil — applied to marine diesel inboards, Yanmar, MerCruiser-Diesel and Meridian-style reservoir work boats. The full-synthetic base holds film strength across the long idling / low-load hours a trawling or houseboat engine actually accumulates, and the additive system carries the alkalinity reserve those hours demand.
Best for: marine diesel inboards, trawler mains, towboat and workboat operators.
Shop on AMSOILBuilt for the conditions that car motor oil can't handle.
A marine engine sits in an environment a car's oil pan never sees: standing water from post-fog cycles, salt- and mineral-laden air during off-season layup, and runtime measured in hours and seasons rather than miles. AMSOIL's marine lines address all three with the same full-synthetic base stock and platform-tuned additive packages.
Water separates out instead of emulsifying in.
Marine engines see post-fog-cycle water ingestion through the breather and trailering / launching cycles — far more than a road-going engine ever accumulates. AMSOIL's anti-foam and demulsibility additives let the water separate out so it can be drained off; conventional car motor oil emulsifies it into the sump, where it shortens additive life and accelerates bearing corrosion from the inside.
Rust-inhibitor package through off-season layup.
Boats stored through winter sit in a humid, mineral-laden environment that rusts bearings, cam lobes, and lifters from the inside out. The marine and marine-diesel additive package carries an aggressive rust and corrosion inhibitor that keeps internals passivated across the layup interval, so the engine starts clean in the spring instead of needing a top-end refresh after the first run.
Hours and seasons, not miles.
Reservoir boats and trawlers log runtime in hours and seasons rather than miles — a short season on Potholes can pile up 80 to 120 engine hours without ever moving the trailer. The synthetic base stock holds viscosity across those long-drain windows, and the additive system carries the alkalinity reserve those hours demand, so the oil stays in spec across an entire season instead of dropping into acid-wear territory mid-summer.
Drain interval depends on platform and duty cycle — the dealer adapts the recommendation to your actual hours and operating pattern on request. NMMA FC-W certification covers every recommended 4-stroke marine line.
Ready to enroll through the dealer referral?Pricing comes from AMSOIL — not added by the dealer.
Enroll in Preferred Customer