'99 2500 w/ 5.3

[79Malibu]

 

I just purchased a '99 GMC 2500 2wd which was a former fleet truck. It has 150,000 mi and has the 5.3L engine. When I picked up the truck it was very cold and it seemed to start and run great. The truck had really black oil in it so the first thing I did was drain it and put in new 10-30 and a filter. Afterwards when I start it cold I hear a rather loud knocking noise until the truck reaches normal operating temp. It also reads 30 lbs of oil pressure when cold. After heating up the knock goes away and the pressure reads over 50 lbs of pressure. Is this the infamous "cold start Knock I have been reading about here. What gets me is the oil pressure, it seems to be opposite of what I have seen. Could comething be clogging the pickup or is there a check ball that is stuck? The engine seems to run great otherwise and has plenty of power. I got 16 mpg with the last tank which seems to be fair to me.