TOO MUCH OIL PRESSURE

FWIW both my last two 340 engines would hit 100 psi cold. In fact I has blowing opening the releif valve on the accusump - what a mess!
Switching to lower viscosity oil on the current engine solved that.
If with a standard pump and pressure spring in your engine its pushing 65 psi at what 1000 rpm? then it will probably get better flow switching to a lower viscosity.
What to pick?
Base it on the oil temperatures typically seen. If you don't have a oil temperature gage, the water temperature will do.
If the oil is somewhat warm, say 180 to 200 F, then the second number in a multi-grade oil is of most interest.
Say you're running a 10W-40. Then try a 10W-30.

But if the oil is on the cold side, say around 120 F at launch, then consider a slightly lower W number as well.

Lighter viscosity will result in less resistance to flow. So that means less pressure at the pump outlet and more oil flowing through the engine itself. Oil pressure at the top of the engine might be better than with the heavier weight oil.