Mobil 1 OW-40
I go with what the manufacturer recommends. In the CTT, Mobil 1 OW-40 is what is recommended (9 quarts of it) to be exact. In my wife's MBZ, which is supercharged, it recommends the exact same as well.
Of course, my BMW M5, Castrol 10W-60 is recommended from the manufacturer. If anyone knows the m5board.com if you talk oil to these guys you can get in quite a battle on what to use. The consensus goes with what the manufacturer recommends though and that is Castrol 10W-60.
You can have your oil tested by Blackstone Labs. They run all of kinds of tests on your used oil.