I'm at 300 miles. Not sure if the guy that does my oil ever changed it so I wanted to make sure I got it done.Yes, an 80/90 or 90 GL5 gear oil is perfect. I always use synthetic, but whatever you want should be fine. Volume for front differential is 17 oz's and the torque on the frnt diff drain plug (if you're interested) is 24 ft/lbs.
Not sure on your miles thing, but I always do all the fluids together when changing engine oil. Good habit to get into. Maintenance chart says every 2 years on front gear oil and every year on rear diff changes.
You're going to have to drop the skid plate for the front diff change.