39 pickup oil pressure fluctuation

[callbald]

the flathead was rebuilt 300 miles ago and they reinstalled the old oil pump saying it was good. I started to notice pressure fluctuation from 30 to 20 at 40 mph. To double check I installed a mechanical oil gauge and got confirming readings. I have been really paying attention to the pressure. Today twice the oil pressure dropped to 5 pounds at 30 mph and bounced right back to 30 pounds. I changed from 30 to 40wt oil today and saw the bounce again. I have been driving in 40-50 degree weather.