That's right, Sticks. 75W-140 will lower the mileage or slow down the truck or both...if the truck is spec'd for 75W90 or 80W-90. I tried the 75W-140 in my old '91 4WD Toyota truck, and it actually couldn't go as fast on the freeway!
I use the Valvoline 80W-90 (non-syn) in my rear diff because the 4Runner rear axle seals are haunted and might not like synthetic. Synthetic 75W-90 everywhere else.

 

Just read the whole thread. Should have made some popcorn.