G'day we always use and recommend to all our customers to use 80w90 in front diff and final drives, we use this as most our customers do alot of heavy 4wd/loader work and we find the ep rating does give longer life to these components, we also change final drive oils at every engine oil change (250 hrs) as there is no where for the contaminants to sump to and they are constantly getting agitated as you drive and after all it is only a couple more litres.
Jon