1980 F100 302,130,000+miles, changed oil and filter,then factory gauge started dropping to the L, installed aftermarket gauges. First start engine, pressure is at 50,after it warms up goes to around 30, when driving oil pressure varies from 0 to 45, after driving at idle, drops to around 15psi, rev engine pressure goes up to 50 or so. Any help would be greatly appreciated.

 

This is completely normal and no cause for alarm at all. 15 psi at warm idle is decent oil pressure.

 

If it was only 30 psi cold and 4 psi hot then it would need a set of rod bearings and a new oil pump,this will reveive old small blocks.