Thanks for the opinions. The oil i was asking about if I could add it to my mobil1 synthetic 5w-40 truck & suv is the mobil1 5w-30 synthetic that says on the back of the bottle is for both gas and diesel. On the back it says ILSAC GF-4, API SM, SL/CF. I was just wondering but I think Im goin to wait till I get some. I like to have it up to full mark when I change . I guess it makes me feel better . I called wally world today to see if they had some they said yes they do. Drove 130 miles round trip to do some business and get the oil......they didnt have it!! I was ....well you know. Not a wasted trip or special trip for it but... Thanks to you guys for helpin This is a great site with great friends.