does he have reciepts for the oil he purchased? that would help a WHOLE lot with that situation...

also, have them prove that the reason for those problems was the oil.

i had a similar situation with the exhaust on my sunfire. the transmission was slamming into gear constantly. took it to the dealer and they said it wasnt covered because the exhaust was causing the transmission to act like that or some bs along those lines. told them to prove to me how the exhaust caused the problem. they couldnt, so the transmission was serviced under warranty. couple family members had this same sort of issue, not saying it will work in this case, but it has for me.