Mark,

I love synthetic oil, and have been using it for years, but one of the
problems with it has to do with switching over to it on high hour, or
milage, engines. Synthetic oil has the tendency to strip away gunk
deposits within the engine. You will note that the oil looks like hell
when you change it for the first few times, that's the gunk being
removed. Now on a fresh engine it's not a problem, but on an older
engine it often results in the engine leaking oil around the seals and
gaskets, you start using oil. And this does happen, I've seen it.

The other issue, and it's not as big, marine oil has more rust
inhibiters in it because of the wet enviornment marine engines operate
in. So you can get some rust inside the block if you use synthetic. But
I will say that the last marine engine that I tore down had been run
with synthetic oil since after break in, and it had no signs of rust
inside, so I'm skeptical of the rust argument.

The 5 weight oil may be too low, you could get some knocking at start
up if you don't have lots of oil pressure (a good rule of thumb is a
350 needs a minimum of 10 pounds of pressure for every 1000 RPM's), not
a good thing. I would go with a minimum of a 20 weight oil.

John