Andre,

Generally speaking, they reccomend straight weight oils because it
tends to hold up better under constant heavy loads. Their's not much of
a differance, but the manufacturer takes the safe rout by reccomending
this. Marine oils tend to have additional compounds in them that help
prevent rust inside the block, but I've never seen it make a
differance. On your flame arrestor, anything from diswashing liquid to
engine degreaser works, I always preferred the degreaser myself. On oil
pressure, general rule of thumb, 10 pounds of oil pressure for every
1000 RPM's, lower than that, you've probably got a problem. I'm not
sure about your grease other than marine grade (water resistant) is the
way to go. Stick to the maintenance, don't beat on the engine, and
you'll be addicted to boating like the rest of us in no time.

Enjoy
John