80W90 is fine in a series 'box, slightly nicer when the box is cold, acts just like EP90 when warm. The thing to watch for is GL4 spec, you need it for the main box. Alot of modern oils are GL5 but thats not good for yellow metals - I think its only the main box that contains them. Transferbox and axles aren't fussy. Anythings much better than nothing so regular level checks are the most important thing.