?? Oil change and pressure dropped??

[Newc]

Background; Bought an [ unknown history] Flathead car with a nice 59ab... high oil pressure showing 45-50 hot, Ok oil was clean maybe the later pump... Then [250 miles later] went in for an oil change used 10-40 conventional 5 qts. usual pressure 45-50 hot. Got about 10 miles and pressure drooped to 30 with 20 idle? Level never changed, oil is super clean. Seems that some thing cleaned out or the valley relief started working? Any ideas barners?? more testing to follow. Newc