Four-stroke outboard oil got you stumped? You are not alone. A question from a reader prompts me to address some conventional oil wisdom that’s probably shared by many boaters. The reader has a new Mercury four-stroke outboard that’s due for its initial oil change. Is it ok, he wonders, to fill the crankcase with the new Mercury 25W-40 Synthetic Blend oil, assuming that the motor had been shipped from the factory with “break in” oil? The short answer is yes—go ahead and use the new Mercury oil. For the longer answer, read on.

outboard oil

Make sure your outboard is filled with an FC-W marine oil that matches the viscosity and spec listed in the owner’s manual.

Break-In Oil

A generation or two ago, engines were factory-filled with a special oil that contained an extra dose of detergent and other additives that helped new engine parts, especially bearings and piston rings, wear in together. The metallic debris from this “break in” would be carried away by the break-in oil and flushed from the engine on that first oil change. But that was before the advent of computer-controlled machine tools, and the very close tolerances of modern engines.

“In most cases the form, fit, and function of new engine parts is of sufficiently good quality surface finish, and good tolerance dimensionally, that it is not necessary to break-in new hardware,” says Mercury oil specialist Frank Kelley.

According to Kelley, new motors coming from the factory in Fond du Lac are filled with the same 25W-40 mineral Mercury oil we can buy in a bottle. Engines smaller than 50 HP manufactured for Mercury in Japan are filled with a 10W-30 oil that’s blended in Japan. The use of a special break-in oil is a practice that has mostly fallen by the wayside.

With a brand-new engine, you still want to follow any operating restrictions during the first hours of use, such as limiting RPM and using variable throttle settings, as described in the owner’s manual. And don’t delay that initial oil change.

“Even with a modern engine and oil, it’s critical not to over-shoot that initial oil change,” said David Meeler, Yamaha marine product information manager. “Think of it as the first pit stop of a long race. For Yamaha outboards that’s at 20 hours, and draining that oil will get any possible debris out of the engine. The dealer tech will also change the gearcase lube and check the magnetic tip of the plug for debris larger than just powder. Then if there’s an issue, it can be dealt with immediately.”

Switching to Synthetic Oil

All modern engines are compatible with full-synthetic and synthetic-blend oils, according to Kelley, and it’s also okay to switch from mineral-based to synthetic oil and back again as long as the oil always meets the viscosity and service specs for that engine, which are detailed in the owner’s manual. In fact, many modern oils are a blend of mineral and synthetic base stocks.

“In the early days of synthetic-base oil technology (the 1970s), those oils had poor seal swell characteristics and they did not solubilize the additives a well as mineral oils,” said Kelley. “It was possible to get into problems with seals and gaskets, and with additive dropout and sludge if you switched back and forth. Today the synthetic-base oils are better made and specific additives promote good seal swell and additive solubility when synthetics are used, so these kinds of problems are much less likely to occur. It’s also perfectly acceptable to use current synthetic-blend oil in older engine products.”


The new 25W-40 Mercury oil formula is offered in standard and a more-durabile semi-synthetic blend format. The 10W-30 is a better choice for rope-start outboards.

Who Needs Synthetic Oil?

Marine engine manufacturers offer a choice of mineral-based and synthetic-blend or full synthetic oils. Again, you want to first follow the guidelines of the owner’s manual regarding oil specification. There have been a few engines, such as the first-generation Mercury Verado outboards, that required a specific synthetic-blend oil. In most cases, however, mineral-based oil is acceptable.

The benefit of a synthetic-blend oil is mostly related to durability–it will hold up better in the most challenging conditions, such as long runs under load at high speed.

“Our full-synthetic Yamalube marine oil is really intended for the high-performance customer,” said Meeler. “The VMAX SHO owner and some of our 4.2 Offshore owners, who really run their motors hard for an extended period of time. It’s also a good choice for the F115 and F70 motors, which can be run up to 6300 rpm.”

Trolling in cold water might really be the toughest duty your outboard oil will ever see. During extended trolling an outboard may not reach normal operating temperature and even with modern fuel injection and engine controls, unburned fuel in the combustion chamber can condense and migrate past piston rings and contaminate engine oil, a situation engineers call “fuel dilution.” When the motor next reaches its operating temp, that fuel should evaporate as the oil gets hot. But if you troll with a kicker and stow it away to run with a main motor, the oil in the small motor may stay contaminated with fuel. Synthetic oil is more tolerant of contamination by fuel and water condensation. But it’s a good idea to get that kicker up to temp once in awhile.

The synthetic oils costs 25- to 50-percent more than mineral-based oil, but we’re talking about a difference of perhaps $7.00 for a gallon jug. Given the cost of a new outboard, even a little 20 HP motor, why not use the best oil you can buy?

For more information visit Mercury Precision Lubricants and Yamalube.