The design of the new Advantek axles is more focused on efficiency than ever before (read more about it here - https://spicerparts.com/parts/axle/automotive/advantek-axle-technology). The amount of heat and metal generated by these axles when combined with the low capacity of the sump does not lend itself for long intervals between oil changes nor is it wise to use 75/80W-85/90 viscosity in them if you are towing or operating in heavy duty conditions.So then Jeep is saying to run a thinner fluid, in contrast to Dana Spicer to pick up MPG's?!
I run 75W-140 in the rear and 75W-110 in the front (Amsoil Severe Gear). I use 75W-110 because it is the upper viscosity that 75W-90 used to be before API updated the J306 tables some time ago. The front axle should hold between 1.3 and 1.5QTs and the rear axle between 1.4 and 1.6QTs.
Hope this helps!