Put a mechanical guage on it first. With that many miles on it. I would think
there is wear on all the bearings.
It's possible that the sending unit could be bad. Had a lot of them over the
years.
What about a chunk of carbon caught in one of the valves? Also check
for carbon tracks on the sides of the spark plugs. Could be something
very simple.
They will just start leaking at anytime. I bought a car from
a guy years back. He couldn't figure out the oil leak. He thought
it was a rear main or a intake gasket. Bought the car. Changed
the pressure switch and resold it for a good profit.