I remember when Reeves Callaway would only honor warranties on his engines if Mobile 1 was used. Anything else, no warranty. Don't know if that is still true, but I only use Mobile 1. The thing with Pennzoil is this, once you use it, do not switch to another brand. Sounds funny but if you go to another oil, you will start to get a sludge build-up. I believe that Quaker State is the same. This is due to the high parrafin content. At least this is what I have been told...