See: http://www.whnet.com/4x4/oil.html for approved oil.

If your going to use Mobil 1, 15w-50 or 0w-40 weights should be used.

YES, you can change the oil every 7500 miles instead of every 3000 miles. I would change the oil sooner though (3,750) for the first change or two if this is the first time synthetic oil has been used in your engine.

Chris