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 that are not found in motor oils formulated for modern engines. 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 in present-day emissions components.

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. Motor oils for gasoline service up to the API “SL” specification (for cars made 2004 and earlier) could be considered. Motor oils with the API SM or SN specifications and newer should definitely be out of the running as the zinc and phosphorus content will be insufficient.

Diesel engines oil with API CI-4 or CI-4+ specification were designed for engines made prior to the 2007 model year. These oils tend to have higher zinc and phosphorus volumes as well as a high detergent content. 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 specification. CJ-4 diesel oils have lower zinc and phosphorus levels.

Avoid Some Marine Engine Oils

NMMA FC-W Catalyst Compatible LogoThere is one tell-tale sign visible on the label of some marine engine oils that may disqualify it from being suitable for flat tappet cam engines. If you see the logo (left) stating “NMMA FC-W Catalyst Compatible”, there’s a very good chance that this oil is not compatible with your flat tappet cam engine. High amounts of zinc and phosphorus can foul catalytic converters in newer marine engines, so any engine oil meeting the “Catalyst Compatible” criteria likely has used a different approach to wear control. This doesn’t mean that these oils are of lower quality, it simply means that they aren’t great for older engines with flat tappet cams.

A logo stating simply “NMMA FC-W Certified” alone without the “Catalyst Compatible” designation may be fine, but one must determine the zinc level from the oil manufacturer. NMMA stands for National Marine Manufacturers Association and “FC-W” means the motor oil is certified for 4-stroke marine engine use.

AMSOIL Synthetic Heavy Duty Diesel & Marine Motor OilOur Oil Recommendation For Mercruiser 5.7L Engines With Flat Tappet Cams

We sell an oil that we believe is perfect for these engines. AMSOIL 15W-40 Synthetic Heavy Duty Diesel & Marine Motor Oil contains 1377 ppm zinc and 1267 ppm phosphorous making 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 an anti-corrosion package for marine conditions. We believe that this engine oil is a great fit for older inboard Mercury engines made by GM.