Compression Ignition Addict
Join Date: Sep 2007
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Boy have you opened a can of worms! There are countless threads on the oil weights to use ad nauseum!
If you search these threads, you'll find anywhere from 15w-40 to 5-40 and all points in between. Some of the answer depends upon your climate and how you use your truck. In a colder climate, the 5-40 oil (like Rotella T6) will help you out with easier starting, smoother running when cold and even fuel mileage. Some prefer the Rotella T5 which is 10-30 in weight.
As for me, I am currently using Amsoil 15-40 but will be switching to Rotella T6 the next go round.