I have talked with a few trans "Rebuilders" and the consensous from them is if it is "New/Rebuilt" start off with your choice of acceptable fluids(10wt, 15W40, Dexron, etc.), but if the trans is NOT new then it is best to continue using the same fluids that are in it as that is what is absorbed into the clutches and CANNOT be removed from the clutches. They have let me know clearly that a fluid change later in life can possibly lead to failure and to lessen that possibility of that is to just stay with the fluid it started with i.e. engine oil or trans fluid. The previous comment about the boiling points of the fluids is correct in that ATF(Dexron, Type F, ATF+4, etc.) is lower than engine oils(10wt, 15W40) and that is why those oils are used in hard on/off road applications such as our military trucks. This is exactly why I will stay with 10wt in my trans. Now in the end it is your truck and you can do as you wish with it. I hope this is helpful.