Synthetic gear oil for top off only or can it be used 100%?

On the bottle of Mobil 1 Synthetic 75/90 it talks about ';top off';. While the bottle of conventional 80/90 talks about ';makeup'; of gear box. Can Synthetic be used in the differential at 100%? Go into a 99 gmc suburban 4x4.Synthetic gear oil for top off only or can it be used 100%?
Most times the label on synthetic oils will state that it is compatible with all other oils. You can use synthetic for a top off or make up in the differential. I would hedge my bets by using the same name brand( i.e. all Mobil, or RP,etc.). There are some differences in additive packages. You will, of course, get the greatest benefit of synthetic if you use it at 100%.Synthetic gear oil for top off only or can it be used 100%?
I use 100% royal purple gear oil in my Harley and works great, and it gets hotter than anything you drive.
I wouldn't mix oil or gear lube as some might have a reaction and cause damage??
yes, it can b used 100%, but u'd have to drain the old first , of course.

bloom-