Just bought a used 99 Surburban 177K on the burb no oil leaks. Starting to go through and cleaning & fixing small things. Went and changed my oil and now have an oil leak on the back cylinder passenger side. Before I changed the oil had 40# of oil pressure, I put in Castrol High Mileage in with new fram filter & have 42# at idle & 50# at higher rpm now, then the leak started. The drip is a drop every few seconds, is the head gasket bad? It's hard to see up in that back corner. Any ideas why the change started after a oil change?