ive got a 1995 series 60 detroit i bought used.miles unknown.until last month the oil pressure was constant at 51 pounds when warm.it might drop a little when id pull a hill.suddenly its dropping below 48 and when it does my engine gets hot.i had an engine oil analysis done that showed good viscosity and normal metal wear.i changed the oil and the overheating went away.another 5000 miles and the same thing is happening to rotella 15-40.just to see what happens i added a fresh gallon of oil.the pressure and temp returned to normal.500 miles later im back to the same problem.with no evidence of contamination or failing engine parts im perplexed