2016 ram 1500 express 3.6 not sure what fluid to use and if I have to do the limited slip additive In the rear any help would be great thanks in advance.

 

I would use 75/140 for both.

 

Thanks for the quick response I read 75 90 for front what's the difference between 75 90 and 75 140? will it ruin the differential if I use 75 90?

 

I think they call for the 75/140 for 'heavy duty' applications, if you tow/haul a lot. The 75/90, or 80/90 will keep your gears happy for the foreseeable future.

Get a build sheet for your truck, and see if the LSD is listed, if it is, you need the friction modifier.