Fluctuating oil pressure query

Guys, thanks for the responses.
I'm using a mechanical guage. 3,000rpm gives me about 75-80mph (3.23 gears). I think that the windage suggestion could be right. As i said, i can run the car in my driveway at 3,000-4,000rpm and it holds steady at 60psi.
I'm using 15w40 oil (not 12w40 whoops). What oil do you guys run?
The motor is a mild tuned 360 (approx 300hp).