owners manual calls for 75w for the NV4500 and 90w for the rear axle. Amsoil says to use 75w-90 synthetic manual transmission and transaxle gear lube. Yes or No? Use other brand?
Amsoil says to use a 75w-90 or 80w-90. Yes or No? Use other brand?
Thanks.

 

I have used the 75-90 for years in both trans and rear. Works fine.

 

Using anything but OEM Syntorque in the NV4500 is at your own risk. Even though guys claim they've had no issues using other lubes, there's plenty of proof that there is/can be problems running anything else. Reason its such an issue is because, unlike regular brass syncros, the NV4500 uses carbon fiber composite syncros and they need whatever is in the OEM lube to keep them happy. If the OEM Syntorque is to spendy then know that you can also get it from GM because GM also uses the NV4500, and they sell it for less than Dodge.

As for differential weight lube, that depends on what you tow and the average temp where and when you drive/tow. General rule is light to no towing then 75-90 is fine, tow medium then 75/85-110, tow heavy then 75/85-140. Tow in triple digit heat and you may want to run the 75-85/140. Live where sub zero temps and cool summers are normal then 75-90 or 110 would probably be best. But no matter what you choose, I'd run synthetic in the differential simply because of the amount of torque the Cummins can deliver to the drivetrain.