From boats.com site
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 SHOowner 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?