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Synthetic Oils for Marine Engines

Amsoil 10W40 marine engine oil Mercury Quicksilver synthetic Marine engine oil

Should You Fill Your Marine Engine
Crankcase with Synthetic Oil?

Are synthetic lubricants a good fit for marine engines? To help you decide between conventional or synthetic oil, let's first consider how oil pumps life into an engine.

Over the past decade synthetic oils have evolved into a very interesting alternative to Arabian crude. It's true synthetic lubricants and gasoline are nothing new. Hitler's petrochemical wizards created them during the Second World War in order to keep Messerschmitt fighter planes and Wermacht Panzer tanks blitzkreiging across Europe and Africa. But what about lubricating today's marine engines with synthetic motor oil?

Marine engine oil performs four basic functions; lubricating, cooling, cleaning and sealing. It lubricates by providing a thin film of oil that buffers rod and main bearings from the crankshaft journals. Further, motor oil's slipperiness also reduces friction.

Oil cools an engine's moving parts by absorbing friction- generated heat and carrying it away with the oil flow. It's interesting to note the water/antifreeze coolant system provides but sixty percent of the engine's cooling. The balance of the heat is carried away to the crankcase sump.

Besides carrying away heat, the flood of oil washes rod journals and valve lifters clean so abrasive particles don't scuff the relatively delicate surfaces. Here's where that detergent rating struts its stuff. The detergent disperses contaminated particles, and keeps them suspended in the oil. When the oil's changed, the particles drain out with the oil instead of building up in the sludge. Finally, oil seals the rings against the cylinder walls, preventing loss of horsepower and fuel economy.

The fly in the ointment is that engine hours reduce the ability of motor oil to fulfill its duties. First, latent engine heats acts like the fire in the refinery, breaking the motor oil down into lighter and heavier molecules. The lightest of the molecules float off into the atmosphere contributing to air pollution. The heavier molecules also create problems by thickening and transforming into sludge, gum and varnish.

But because they are not mineral based, as are conventional motor oils, synthetic lubes resist this mini-refining process, and the resultant break down of the oil. And because synthetics are slipperier than petroleum based lubricants, less energy is lost to friction. Less friction means less heat. Synthetically lubricated engines typically run from 5 to 20 degrees cooler than their counterparts. That results in less engine wear, more available horsepower and up to a ten percent improvement in fuel economy. Besides superior lubrication, the most prized virtue of synthetic oil is that oil changes can be extended from four to eight times the norm.

There are, however, certain disadvantages. First, is price. Synthetics cost from two to three times that of the conventional mineral oils. Additionally, synthetics shouldn't be used in an engine that burns oil. Obviously that's because you'd be burning up very expensive oil, but additionally because engines that burn petroleum based oil tend to burn even more synthetic. Moreover, if the engine already leaks precious conventional oil, it will leak even more synthetic.

Some say the complications are even worse for boats and RVs whose engines are infrequently run. Naysayers cite tests that have allegedly shown automotive synthetic oils don't provide adequate rust protection for mothballed boats and motorhomes. This fable is not true, but it's understandable that the story at least got started.

The first generation of synthetic oils were used for racing, and were based on glycol. They had virtually zero ability for preventing cylinder wall rusting. However, the second, and current generation of synthetics are exemplary at inhibiting rust. Additionally, marine motor oils are packed with additional additives for rust prevention.

What about the argument that marine and RV engines sit idle longer and the acids just sit there eating up engine parts? Easy answer. The synthetic oil manufacturers blend additives into the synthetics that neutralize crankcase acids, and thereby provide protection touted as superior to conventional motor oil.

However word of caution. If interested in pouring a can of synthetic oil into your outboards injector reservoir or six gallon tank, first read the label. Look for the words "BIA Certified for service TC-W". BIA approval certifies the oil will provide the kind of lubrication your engine needs. And regardless of whether you use conventional or synthetic oil, fog the cylinders when winterizing to protect them against rust. Amsoil is specifically blended for marine engines hard use and need for rust prevention. Similarly, Mercury Marine blends 4-stroke oil tough enough to survive the punishment thrown it by its turbocharged Verado outboard motors.

Synthetic lubricants are also available for the outboard and stern drive gearcases, as well as for RV and tow car manual transmissions and rear ends. These premium lubes are touted as able to provide extra rust and wear protection for the gearset, all the while reducing drag and improving power output and fuel economy.

Sounds great for RVs and pickup trucks, but probably not a wise choice for a powerboat. Recall how a synthetic lubricant is designed to provide its benefits over the long haul. Now consider that an outboard motor or stern drive's gearcase should be drained and refilled at least twice a season. So why bother to grease the gears with expensive high tech lube when it's barely going to twirl around the gear teeth long enough to catch its stride? Our advice; great for a car or motorhome, maybe not so hot for outboards.

potrait of tim banse

Marine Engine Technician, Timothy Banse, has published articles in Popular Mechanics, All Chevy, Pickup Van & 4-Wheel Drive, Mecanica Popular, Motor Boating, Yachting, Mar y Vela and many other magazines and newspapers from around the world. He writes about cars, trucks and tow vehicles as well as automotive- and marine-engine technology.