I tried the new 10W30 Synthetic Blend Rotella in my truck and i like it very well. Truck ran great on my 400+ mile weekend trip and at 16 bucks a gallon at wal-mart not to bad of a price.
I dont tow with this truck and dont haul much with it anymore either because my trailer is a gooseneck and i didnt want to mess my rihno lining up. I use my grandpaws stroke for that.I wouldnt use anything under the 15-40 thats reccomended. Do you know what oil actually dose? Pulling a heavy load the crank and other components need the oil to give a clearance between engine parts. A heavy load will cause more strain on a motor and with a thinner oil it could cause internal parts to collide causing friction and :blowup
Pull a 10k trailer for a few hundread miles and see how it goes. Hope you got a warranty :drink