I have a 51 Chevy pickup with a new GM 350 crate with only 235 miles on it, a 700R4 tranny. When I start it and run it at about 1500 rpm the oil gauge (new Classic) will bounce as much as 45 lbs. I hold it at 2000 rpm and the pressure sits at about 90 lbs. When I drop it to 1500 rpm the pressure drops to 75 lbs. At that point it will vacillate between 65 and 90. Sometimes the movement is erratic and sometimes it moves around slowly.
I have air but this is when nothing is on. I worry there is something wrong with the oil pump.