Know a couple of guys around home that are running new Ford Super-Duty's with both the 5.4 and 6.8 liter Triton engines. Thing is, they tow horse and cattle trailers all the time, and run the heck out of their trucks. I'm very curious about the 5w-20 oil that you have to run in those Fords now. How does this stuff protect under high-stress operating conditions? Would it really hurt to go with a 10w-30 (such as Castrol GTX or Chevron Supreme), or for that matter, even a 15w-40 (if temperatures weren't too low)? Is there any reason Ford went with this rating besides their continuing passion to obtain higher fuel economy numbers?