my friend just gave it to his son, he bought it brand new, has 75k miles. Manual says 5W30, 10W30, or 10W40 can be used. his son drives 5 miles to and from school,and they use it to pull a 30 foot fifth wheel camper, so the engine gets a good workout (pulling the camper). From what ive read on the titan forums, it seems alot of guys are running GC and Mobil 1 0W40 with good results. So out of GC, Castrol 0W40, and Mobil 1 0W40, which would be the best to use?