What type of Differential oil?

Well, I've done rear axle services on vehicles I know still had the fifty year old plus conventional 80w90 in it and the gear oil looked fine and the gears were all in great shape. If conventional gear oil lasts that long, it's good enough for me. Well over 90% of yall will never put your cars even close to needing synthetic gear oil, so it becomes a total waste of money, but knock it out.