If a fluid weight is not compatible with the diff would it be obvious? Would it clunk n make noise? Like how redline mt-90 wasn't wasn't compatible with the trans n people were having issues with it?

 

The trans situation is very different and much more sensitive to rating due to synchos and abundance of gears. The MT90 (redline) was a bit too thick and some reported *notchy* shifting..so some started to BLEND the the MT90 with MTL 70/80 and that worked very well. They now have MT85, which was made for the Nissans. OK....the DIFFERENTIAL is a different animal..beefier components and not so sensitive to small WT differences. That's prolly why Nissan never used their *own*? In fact, I really believe that a very good 75/90synthetic (like M1) is actually SUPERIOR to a 80/90 Pennzoil..even though it has a slightly LOWER range rating. Just look at the testing I showed here. Bottom line..this is NOT like the tranny problem where a HIGHER rated oil was being used.
Unlike the TRANNY, with the MT90 problems, there are MANY people who are using 75/90 (GL-5) in the DIFF with ZERO problems..in fact I haven't read about a single one in the 350Z..G35/37.