Sounds to me like you have water getting in the engine.

I personally would do some tests, before just going out and beating on it, trying to get oil temp hotter.

Thousands and thousands of black mercruiser engines running around, without factory oil thermostats, or oil temperatures above 212, and no milkshake on their stuff.

If this was a fairly recent occurance, I'd start looking at a possible leaking intake gasket, corroded intake manifold, exhaust manifold leak, leaking exhaust guide, and so on.

riser gaskets ?

good chance it has a cracked or rotted exhaust manifold causing some reversion.it could be pushing water past the valve guides and that shows up in the valve cover first.

change my opinion - riser gaskets - or bad valve(s) causing water reversion. or what Mike said above.

I would pressure test exhaust / risesr still bolted together and do a comp test / leak down test

whats your spark plugs look like ???