Due to bad luck, Pennzoil would never be in any engine that I own, This goes back many years and maybe they changed their formula(parifin)I have about 1000 miles on my new engine (probably more)I have a change of Rotella 15 W 40 heavy duty ready to go in. From what I have heard from many people, and I think Yellowbird was one this is the oil to use in our flat tappet engines?????