My high milage engine starts great, but when it gets warm the oil pressure drops completely , and lifters are knocking hard. Then I can pullover for just a couple mins , fire it up and its fine. Is the oil pump the only possibility?

 

welcome to FTE.
sounds like a plugged oil filter, very low oil level in the engine, or an oil pan that is full of crud. if it was the oil pump, it would not get oil pressure back after sitting a few minutes.

 

Pick-up screen on the oil pump is blocked with sludge........bet on it.