Low oil pressure makes me nervous

On a road trip now '72 318 auto. Changed out my Castrol high-mileage 25W-60 wt. (not really available in the USA) for straight Quaker State 40 wt. before I left La Paz, Mexico. With the 40 wt., factory oil pressure gauge reaches the second graduation (near the center of the gauge) when cold (40 PSI?) but drops to the first graduation when warm (20 PSI?). Driving north up the Grapevine on Highway 5 in California this week, the gauge dropped to halfway between zero and the first graduation (10 PSI?). I was waiting for the motor to freeze up but it just kept climbing the grade and the pressure went back up to the first graduation on the way downhill. I continued another 300 miles to my destination to where I am now (Mill Valley near San Francisco). After a search, I found the 25W-60 not available in California at Napa, Autozone, nor Oreilly's). I just ordered online some Castrol 25W-60 online for my return trip and hope to get the oil pressure up a little more.

The good news is I am averaging 16-17 MPG in a 1972 Satellite Sebring Plus.