Marine Engine Oil: Synthetic or Conventional?

When you’re talking about engines, gas or diesel, inboard or outboard, you can always find someone to argue that your boat or tow vehicle would better off using conventional motor oil. Or that it would better off if you used synthetic.

We have heard all the arguments. Heck, I was debating the topic along with everyone else. Then it occurred to me. I’m a journalist. I can get an expert on the phone, tell him I’m writing a story about motor oil or gear oil and get answers from a person far more knowledgeable than your Uncle Ned who swears by the Sinclair single-weight castor oil he used in his ’39 Hupmobile.

So I did. In fact, I wrote a number of stories on oil, and rather than making you wade through thousands of words of copy, let me give you the CliffsNotes version.

High-quality synthetic oils like Mobil Delvac, shown here filling a fuel filter for an Isuzu diesel, don't build carbon deposits the way standard motor oils do. Photo: Doug Logan

For engines, synthetic is better. Here’s why. Conventional motor oils can lead to carbon buildup on pistons, in the combustion chamber, and on the back of the throttle blades, all of which can lead to engine noise, reduced power, and drivability issues.

During the combustion process, engine oil sneaks into the chamber through the PCV system and in the form of blow-by from the crankcase. Since conventional oil is rich in carbon, deposits form. Carbon, which is nature’s molecular magnet, cakes beautifully on pistons, valves, throttle blades, and  combustion chambers. And when engines face severe loads, such as pushing a boat, it only makes things worse.

Since engine manufacturers have really gotten a handle on fuel delivery, engines now run consistently at an optimal air/fuel ratio. That means there are no lean periods to help burn off carbon buildup, as would happen with carbureted engines. “Take ‘er down the lake at wide-open throttle and blow ‘er out,” was often good advice for cleaning carbon deposits. Not any more.

Because the air/fuel ratio is always optimized, that no longer works. Only chemicals or friction will remove carbon deposits. Synthetic oil, which has no carbon, leaves no deposits. It’s a bit more expensive, but depending on how much you use your boat, synthetic engine oils can last an entire boating season.

For gearcase lubricants, experts I’ve talked to argue in favor of conventional mineral oils. Why? They pull heat out of a part better than synthetics, which is critical because lower units and gearcases typically have no fluid cooling systems. The good news is that conventional gear oils are less costly than synthetics. So, unless the manufacturer dictates otherwise, save some money and go with a conventional gear oil, and be sure to follow the manufacturer’s recommended change intervals, and you should be fine.

Now you have some material for next time Uncle Ned pipes up.

Brett Becker


  1. Ole Ohm Lundager says:

    Which kind of motoroil will you reccomend for a Yanmar YSE 8 , one cylinder dieselengine


    Ole Ohm Lundager


  1. […] unless they come up with something better. But in the end you have to decide which is best for you. Marine Engine Oil: Synthetic or Conventional? | Boat Trader – WaterBlogged Bernard 2012 Pheonix 921 – Yamaha 250 SHO Reply With Quote […]

Speak Your Mind