Re: Wrong oil?????
All you have to do is decide who knows more about your engine, you (or some random poster on the internet ....) or the hundreds of engineers at Mercury. Should be pretty simple to make a decision after you settle that.
Mercury recommendations:
Notice how the last recommendation, #5, is for straight weight automotive oils? There's a reason they don't recommend multi-grade, light-duty automotive oils for boating use:
Simple lesson on multi viscosity oils
First you have to understand how multi-viscosity oils are made. Since the 50's, that's been done with "viscosity index improver" additives, a polymer (long chain molecule, also known as "plastic"). What happens is this polymer expands as the oil gets warm, taking up more space between the oil molecules, effectively making the oil "thicker" than it would be without the additive. So, a SAE 5W-30 oil would be SAE 5 without the VI improver, SAE 10W-30 would be SAE 10 without it, etc.
The first thing to break down in an oil is the viscosity index improver. When the oil is under load, the plastic VI improver molecules are sheared. They are no longer able to take up as much space when the oil heats up, thus the oil slowly reverts to it's base viscosity. IE, 5W-30 slowly becomes 5 weight, 10W-40 slowly becomes 10 weight. The more load on the oil, the faster it breaks down.
How do you get a 5W-30 oil to be a 5W-40 oil? By putting in more plastic! GM had to warranty a bunch of diesel engines due to VI improvers gumming up their roller lifters, so they banned the use of high viscosity index oils (like 10W-40) in their diesels, and carried that over to their gasoline engines for general principles.
The "typical" synthetic doesn't use as much VI improver as the same rating petroleum base oil, so it's "typically" safe to use a 10W40 synthetic oil in an engine that calls for 10W30. That would only apply that to the "premium" synthetics, though, like AMSOIL, Mobil 1, and Redline. Don't trust any of the other "majors" synthetics (Pennzoil, Castrol, Valvoline, etc.) for this property.
Fact is, your boat will probably die from a bad impellor or bad exhaust manifolds LONG before you have to worry about any oil-related issues.
