I've been slowly bumping up the weight on my '61 Impala for this reason. Pressure cold is fine but when hot it will drop below 10 per the mechanical gauge in the car.When hot, with the 15w/50, its about 18 psi idling, tested with a mechanical gauge.
Dont remember what it was with 5w/30.... but it was pretty low. Low enough to make me jump straight to 50 wt!
Dont think its the bearings. It's very quiet at start up.