Found this, might help
Copied part of a thread on "Regular synthetic oil vs Marine synthetic oil use"
from here
http://www.bobistheoilguy.com/forums...Number=1177482
The Outboard Expert: Oil Essentials
Marine oil standard is the formula for long engine life.
by Charlie Plueddeman, March 13, 2007
Boats.com
In my last column, I mentioned that Yamaha has introduced a new 20W-40 version of its Yamalube 4-M, a four-stroke oil that meets the NMMA FC-W standard. Oils meeting FC-W have been on the market for almost two years, but I think there are still many outboard owners who are unaware of the FC-W rating, and why it's important to use this oil in a four-stroke outboard. So here's the short story on FC-W.
Oil that carries the diamond-shaped "NMMA Certified FC-W" label has been designed specifically for use in marine engines, and meets minimum performance standards developed by the lubrication gurus at the National Marine Manufacturers Association Oil Certification Committee. I've met some of these guys, and they really are smart. Like they can recite the periodic table of elements from memory.
Until FC-W, engine manufacturers have specified the use of a high-grade automotive engine oil for four-stroke outboards. But in recent years automotive oil formulas have changed, as car and truck manufacturers put more emphasis on reducing internal friction to improve fuel mileage. This made auto-spec oil less desirable for use in marine engines, where operating conditions are significantly more challenging. For example, how often and for how long do you run your car or truck at full throttle? Maybe for a few seconds as you accelerate down an on-ramp. Your outboard, on the other hand, may spend a lot of time at WOT. Even when cruising, an outboard is under relatively heavy load and at least 75 percent throttle while it operates in an environment that includes high humidity and salt air. Compared to a car engine, an outboard leads a tortured life.
In designing the FC-W formula, the NMMA committee sought to address two key issues. The first is resistance to "shearing" during high-rpm and high-temperature operation that rarely occurs in an automotive application. Under shear stress, oil can become thinner and lose its ability to separate moving parts, leading to accelerated wear or even catastrophic failure in extreme cases. The FC-W spec imposes a higher minimum viscosity level, which means that FC-W oil will not thin out as much as automotive oil of the same viscosity under the same heat and rpm conditions.
The second issue addressed by the FC-W standard is corrosion. Obviously, an outboard lives in an very humid environment that can also be salty. When the engine is shut down, air remaining inside the engine can condense on bare-metal surfaces and this moisture can either attack the metal directly or become mixed with the oil and circulate through the engine. The FC-W spec adds a rust-inhibitor requirement that does not exist for any automotive engine oil and will better-protect vulnerable surfaces like valve springs and cylinder liners.
