A week after I bought my 2001 HO 6 speed, the Amsoil guy approached me and asked if I was thinking of using Amsoil...
He said all the right stuff, like the transmission and rear end will run 30F cooler than with the stock grease, etc... ( I have trans temp and rear end temp gauages )... he also said that the $12 per quart oil in the transmission was nothing but synthetic 5w40...
Well, I was convinced and we drained everything and put Amsoil in, top to bottom...
After 100 miles, my temp gauges were exactly where they were before the Amsoil, AND, IT DAMN NEAR TORE THE TRANSMISSION OUT OF MY TRUCK !!!!
I made the Amsoil man come back and put all the factory grease and oil's back in it...
I change my oil every 3k, oil filter every 6k, fuel filter every 9k, transmission and transfer case every 24k, rear end every 12k ( because i do nothing but tow, grossing 17k ) and I clean and re-oil my K&N air filter every 12k...
It's cheap insurance and my truck will last well beyond the average 400k useful life...