Re: automotive 80w90 gear oil vs marine 80w90 gear oil. is it only $$$$
The whole 'marine doesn't foam' thing doesn't apply to a GL-5 rated oil (GL-5 is also non-foaming.) Check it out on the wikepedia, or any site that breaks down the differences in the GL ratings.
As far as water/oil separation. They say that the marine oils do not allow that water and oil to mix, but at the same time, when I've had leaky lower-ends in the past, my GL-5 automotive 80/90 didn't allow it to mix either! Water would always drain first when I cracked the screw; it was never mixed in.
So anyway, who knows. They have NEVER released proof that their claims of marine lower-end oil containing these special properties are true. The local boat shops here also use Automotive GL-5, and I have for years as well. I stick to the 'if it ain't broke don't fix it' rule. They say "cheap insurance" - but marine oil ain't cheap, and I don't believe in the benefits of it at all, so I'm not even considering it 'insurance.' To me, cracking the drain screw every few outings is much better (and free) insurance. I mean, if you're staying on top of your lower-end (no leaks), then you've ruled out the single only valid 'plus' of marine oil (assuming you trust their claims) - the extra protection should water get in.
