what would the most likely culprit be for a loss of oil pressure at warmup?

10psi at idle is actually within specs. It is normal to have lower pressure hot than it is cold due to thinning of the oil as it gets hotter. Otherwise normal causes for low oil pressure include oil pump output, and internal bearing wear. (crankshaft and camshaft)

How many miles and how much sludge in crankcase and on pick-up screen?

Sludge in oil is a common cause.. Cleaning the pan and replacing the oil pump usually fixes the problem..