I believe you have mentioned the Prolong before. Are you using the engine treatment or oil stabilizer? It's not on the shelf anywhere around here. Do you order online? Is the M1 also 40wt? A couple of D2 owners swear by Royal Purple. I think Im going to stick with what I have been doing.

I'm also concerned that re formulating involves diluting Zinc levels even further. When the T4 hits the shelf I may begin using the Zinc additive.

 

I use the engine treatment. I got several bottles when a Auto Zone was clearing them out a few years back. Amazon sells it in gallon jugs for pretty cheap, which is what I'll be buying the in the near future.

The M1 TDT is 5w-40. I'd be just about as comfortable using the T6. The only reason I prefer the TDT is because it's a Class IV synthetic, which means it was designed from scratch to be a motor oil, as opposed to a natural byproduct that was refined to work as a motor oil. Does it really matter? Probly not.

Royal Purple and Amsoil make amazing products, but I just can't justify the cost. I ran Amsoil for a while and quit when I realized that after shipping costs it was about twice as expensive as M1. I just can't believe it's twice as good of an oil.

You bring up an interesting point about the ZDDP being diluted. I was looking at virgin oil analyses earlier, and was surprised to see that Chevron's Delo 5w-40 has a bit more ZDDP than the T6 or M1 TDT. That said, I think these trucks have too much oil in them anyway, especially with the mondo filter some folks run. I'm not too worried about dilution.