Well I'm now runnning 5W30 Pennzoil in my truck 5.3L (146000km). The oil pressure is below 40 psi at idle and about 48psi at 2000 RPM. It seems to me the deterioration of the oil, the oil has caused this, 3500km on it and I didn't notice this before. Previously I used Esso XD3 0w30 and had oil pressure always above 40 and just shy of 60 at 2000RPM, even with a winter front installed. I was more comfortable with these values. Oil pressure started to go after nearly 7000km on this oil. I'd like to go back to the 0w30 or even a 0w40 I don't know. Can the XD-3 0w30 be used year round? I've seen this before with the crap oil pressure and 5w30, what do you guy's think? I'm suspect of a 40 wt. since the owners manual clearly stipulates not to use 10w40, then you read the EPA documents regarding GF-3 and you see that manufacturer's are mandated to persuade vehicle owners away from oils not GF-3 by using strong language in the owner's manual. I get confused. Anyone own a 5.3L GMC and found an oil that engine really goes for?