As the engine ages the pump wears and pump clearances open up so that
it pumps less volume. The bearing clearances also increase so oil
flows through them easier. The result is a drop in oil pressure.

I've seen that in my A-65 and others. The original potser had had
his bottom end redone, so unless the mechanic paid no attention to
crank wear and put things together with big clearances, I'd say that
the accessory cover needs to come off and the pump fixed. The most
likely wear is the aluminum pump cavity, since it sucks unfiltered oil
and is easily scored up. If it's a homebuilt the cover plate could be
lapped and the pump housing face machined lightly to reduce the
endplay. If there's serious scoring the whole accessory cover needs
replacing. And where do we find a new A-65 cover these days?

Dan