Got a GM 5.7 with about 225,000, but its needs a good cleaning. Currently what is the best detergant oil on the shelf? I dont want to get real aggressive, the previous owner was using oreilly store brand oil, not sure how it stacks up. Im sure its not bad, but prob not the best either. I dont expect expect miracles but anything that can help clean it up will be good. Have seen a lot pointing to pennzoil? Never been a big fan cause it seemed like it used to and sludge real bad, but I guess thats changed. Ran a 3.8 since new with mobil 1 and at 160k it looked like new inside, almost like it was fresh from machine shop.