Engine Room

By admin ~ February 2nd, 2012. Filed under: Engine Room.

Engine Oil Explained

Marine engine oils offer important differences.

 By Doug Thompson 

When it’s time to change or add oil to your boat’s engine, it may be tempting to use automotive oil simply because it’s cheaper and easier to find. Engine oil is engine oil, right? Wrong. While any oil is better than no oil, unless you’re stranded on a desert island, you have time to prepare. And here’s where the National Marine Manufacturers Association (NMMA) has helped with a certification program for marine engine oil for two- and four-stroke gasoline engines.

First, a quick explanation of what oil does within your engine. Oil provides lubrication between the metal surfaces so they don’t grind themselves apart; it minimizes exposure to oxygen; it also takes heat away during the combustion cycle. Further, oil collects the after-effects of combustion and suspends it for removal when you change the oil. That’s why, when you change your oil it is dark in color, the result of suspended silicon oxide and acids.

There are definite differences between automobile engine oil and marine engine oil. Marine engines are run at higher loads and RPM’s than car engines, and there’s also a much greater chance of rusting due to the marine environment. Marine-formulated oil has the advantage over automobile oil because special additives for marine oil prevent the thinning of oil at higher RPM levels and include rust inhibitors.

“The engine manufacturers came to the NMMA to establish oil certification by brand because of the increase in warranty issues,” said Tom Marhevko, Vice President of Engineering Standards for the NMMA. “We update the list of approved oils about every two weeks and the brands are re-registered every year. We do that because oil distributors and manufacturers are constantly changing.”

You can find the list of approved engine oils at: According to the NMMA, a TC-W3 approval stamp will be on the label for approved oils for use in two-stroke engines, the FC-W stamp on labels for four-stroke engines, and the FC-W(CAT) stamp on labels for four-stroke engines that have an exhaust-treatment catalyst system.

Interestingly, the NMMA does not have a certification program for marine diesel engine oil. “We’ve never had a request for diesel oil certification from the diesel manufacturers,” Marhevko said.
However, the certification for marine gasoline engine oil has greatly simplified life for the consumer. The best advice is to simply choose an engine oil that carries the proper certification for your engine type. That’s also going to be in line with what the engine manufacturer recommends.

For example, both Mercury and Volvo Penta have branded engine oils they recommend: Mercury’s Quicksilver two-stroke oil; MerCruiser four-stroke oil and Quicksilver diesel oil; and Volvo Penta-branded oil for four-stroke and diesel engines.

The manufacturers’ guidelines will also help you choose what type of oil to use. There are three types of oils: mineral, synthetic and synthetic blend. Synthetic and synthetic-blend oils cost about twice as much as mineral oils but that doesn’t necessarily mean they are better for your engine. What’s most important is checking the level of your oil on each outing and changing the oil regularly. Boat owners often let the boats sit for weeks or months at a time without use and then expect to just fire up the engine without any problem. Also, with new engines it’s important to follow what the manufacturer recommends to stay within warranty guidelines. Engine oil should be changed annually at a minimum after the engine’s break-in period.

For its gasoline engines, Volvo Penta recommends using Volvo Penta 10W30 mineral oil (catalytic converter approved) for the first 50 hours or annually, and then changing to its 10W40 Full Synthetic Engine Oil. This new oil has an enhanced additive package offering the highest levels of corrosion protection, superior lubricity, high viscosity index for easier starting, less engine wear and performs better across a wider range of temperatures than traditional mineral oils.

For its diesel engines, Volvo Penta recommends using its Volvo Penta VDS-3 Diesel Engine Oil. This oil was developed for use in harsh conditions under heavy loads and the high temperatures that are prevalent in marine use. It has been tested to Volvo Penta’s highest specifications for purity and protection against wear for year-round use. The oil is specifically formulated with high-performance diesel grade detergency additives, reduces engine wear and combats cylinder polishing.

Mercury offers similar recommendations for its two- and four-stroke engines, as well as for its Cummins-MerCruiser diesel powerplants.

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