I'm the proud owner of a 2000 Chevy Silverado ext. cab 4X4 Z71 with a push button controled transfer case. A few weeks ago I broke 50,000 miles on the odometer and I need to change the transfer case fluid and differential fluid and change the transmission fluid and filter as well. Ive done a little bit of research on the topic and I know that GM recomends using 80W-90 in the front dif. and 75W-90 in the rear. I want to use Royal purple gear oil for both diffs but RP only sells 80W-90 gear oil in 5 Gal buckets which is way too much for the job so would it hurt to use 75W-90 or any other weight gear oil in the front diff??? I also know that if the differentials are lockers that I need to put additives into the lubricant so does anyone know how I can find out what type of diffs that I have??? And if my diffs are lockers what types of additives do I need to add to the gear oil???