Our friends recently purchased a 2008 CTS with the 3.6 engine & they recommend using synthetic "oil". The Enclave 3.6 engine recommends regular oil. Can somebody in the know explain why the difference? I assume the 3.6 engines are the same? I know a lot of people on this forum prefer syn. & I'm sure its better but thats not the question. Anybody know? I did a search on this board & didn't find that this had been discussed.