i just got a truck.is a Ford f-650 and has a 10.4 liter diesel by Caterpiller.it already has twin turbo to boot.the previous owner used 90 weight oil in the engine.although the motor is being rebuilt i just need a good direction to go when selecting the proper motor oil.any good help would be appreciated.

15w 40 would be my best recommendation why so heavy? cause it was worn and leaked or burned a gallon a day?

start if off right when you do select and oil and try this
http://www.sfrcorp.com/product.asp?ProductID=1

original owner said he was using the truck in a mining area,strip mining i think he called it,anyways he said the bed would be full and be running 22 to 35 degree climb to get to the surface.i am considering to use 20w50 oil after rebuild is complete.just a thought.