MY owner's manual for (my 02 Silverado with a 4.8L/ LR4) tells me that the oil I should be using is 5W-30, and that 10w-30 is an acceptable backup. I notice a difference in the two. I notice a seat of the pants in difference in response (manual transmission, so yes I can notice it), and the 10W-30 maintains a more steady pressure, but its a little lower (around 60). The 5w-30 goes near 75 when cold, drops to about 50 at idle, and 65-70 when driving or getting on it. (I AM JUST GUESSING WHEN I SAY: ) to me, that seems like the 10w-30 is too thick, and only flowing as well as the pump is able to. I'm not going to tell you which oil to use, but if I had to choose between going off of a random suggestion online that tells you to guess at it, or going off your owners manual..... Go read your manual..