Not sure about break in but ok to use 75w-90 full synthetic from Brand companies like: Valvoline, Mobile 1, Amsoil, Castrol, Red Line, etc. just use GL5 in the diffy. For diffy I prefer when the label only specs GL5 not multiple GL ratings that appear on some labels. Gear lubes use these multiple GL ratings because they say they are upward compatible but only for diffys but not so in manual trannys (per discussions with Tech in oil companies).
The 75w-90 gear oils are spec'd as full synthetic. Friction modifiers not an issue they are in the gear oil I use and no issues, I use Mobile 1 full synthetic
