mercruiser does not build engines, it assembles marine systems. ask the
manufacturer of the engine for its advice, and then go with the 20w-40 it
recommends. Then replace your mechanic, or be prepared to spend some money
with him

I have Mercruiser 5.7's 1989 and boat on Lake Erie near Cleveland.
My mechanic recommends straight 30wt oil and that's been fine, today he
said a new bulletin came out from Mercury and it was something like 30
wt up to 60 degrees F and 40 wt for over 60 degrees F.

Now my mechanic says to use straight 40 weight.

What do you think?