Correct Oil For Mercruiser 5.7L Flat Tappet Cam Engine?

Updated- May, 2021

A reader asks: I’ve got a 1988 10 meter prowler with twin-inboard Mercruiser 5.7L engines that produce 260 horsepower each. As these engines have flat tappet cams, what would be your engine oil recommendation?

Our Answer: GM produced 5.7L (350 c.i.) engines with flat tappet cams for Mercury up until 1995. Making a proper motor oil choice for these engines is crucial. Flat tappet camshafts require high ZDDP (zinc dialkyl dithiophosphates) levels.

Both zinc and phosphorus are anti-wear additives that have been reduced in the latest motor oil standards. The reason being is that high concentrations of these additives can cause problems with present-day emissions components. Therefore, there are better choices than simply buying 4-stroke marine or automotive engine oils that meet the latest specifications.

A common rule of thumb for zinc levels of motor oils for flat tappet cam engines is a starting point of about 1200 ppm and up. If a motor oil’s zinc level is not in this neighborhood, do not consider it for flat tappet cam engines. Motor oils for gasoline engine service up to the API “SL” specification (for cars made 2004 and earlier) can be considered. Motor oils carrying the API SM, SN, SP specifications (and newer) should be out of the running. The zinc and phosphorus limits of engine oils meeting the newer industry specifications may be insufficient.

Diesel engine oils meeting the API CI-4 or CI-4+ specification was designed for engines made prior to the 2007 model year. These oils tend to have higher zinc and phosphorus volumes. Provided that the zinc levels are determined to be in the ballpark, diesel oils with these specs can be great candidates for gasoline-powered Mercruiser engines with flat tappets cams. Avoid diesel oils with the newer API CJ-4 or CK-4 specifications. They do offer world-class protection in diesel engines. However, these diesel oils do have lower zinc and phosphorus levels.

Our Oil Recommendations For Mercruiser 5.7L Engines With Flat Tappet Cams

There are a few motor oil viscosity options for Mercruiser/GM 350ci/5.7L engines. The 10W-30 grade seems to be the most popular choice for these engines. Others opt for 10W-40 or Mercury’s 25W-40.

Here are our options for GM marine 5.7L engines with flat tappet cams only. We would have different motor oil options for roller cam engines and other inboard marine motors.

10W-30 Option

If wish to use 10W-30 in your engine with flat tappet cams, our best solution is AMSOIL Z-ROD 10W-30 Synthetic Motor Oil. This product boasts 1440 ppm of zinc and 1320 ppm of phosphorus for powerful flat tappet protection.

Z-ROD 10W-30 is marketed as an oil for classic cars and street rods. However, it borrows robust anti-rust/corrosion ingredients from AMSOIL’s marine oil products. Therefore, it is perfectly appropriate for inboard marine engine applications.

10W-40 Option

For those wanting a 10W-40 for their flat tappet cam engine, AMSOIL Z-ROD 10W-40 Synthetic Motor Oil is a great option. Like the 10W-30 version, this product also has 1440 ppm of zinc and 1320 ppm of phosphorus. It also has a very powerful anti-rust/corrosion package.

25W-40 or 15W-40 Option

For those wishing to use Mercury 25W-40 Marine Oil or a 15W-40 oil, we’ve got a great solution. AMSOIL 15W-40 Synthetic Heavy-Duty Diesel & Marine Motor Oil contains 1377 ppm zinc and 1267 ppm phosphorous. This makes it a potent force against flat tappet cam wear. This API CI-4+ Synthetic 15W-40 has a powerful detergent package, extended service capabilities and anti-corrosion ingredients. For older inboard engines, we submit that this is an ideal engine oil.