So the manual calls for a 15/40 on both the zforce 500 and 800 I run that weight in my 6.7 Cummins so is this the same

 It seems most people are using 5W-40. It needs the JASO rating. A lot of us use diesel oil like Rotella T6.

I use Rotella T6. 

I also use 5W40 synthetic Mobil 1 in my 500. This is not a wet clutch system though.

It covers the colder temperatures we have now started for the next 4 months.

Rotella T6 is perfect for our engine.

Rottella T4 and T6 are JASO approved for the wet clutch system

Do not use t4   made that mistake and clutch slipped for an hour till I could get back to the truck and head home and change it

Do not use t4   made that mistake and clutch slipped for an hour till I could get back to the truck and head home and change it

Odd, I run T4 and it's just fine. Same rating as T6 but not synthetic. No slip with my wet clutch equipped 800.