Built onto the inside tip of the diff's drain plug is a small but powerful magnet. It's purpose is to attract and trap any metal shavings that may be created as the differential gears wear. It's not unusual to find a few little shavings stuck to this magnet when you go to change the fluid. You will know if there are too many shavings because there will be a "blob" of them stuck to the magnet when you pull it out of the case.
The automatic tranny pan also has magnets. Stuck to the bottom of the tranny fluid pan are two long, skinny magnets that serve the same purpose, but for the transmission instead of the differential. My friend Matt recently pulled the pan after 105K miles on the tranny, and found almost NO particles on the magnets. The MKIV tranny is tough if it's not overheated. It's highly recommended for auto owners to add a secondary tranny cooler. You can buy a kit from Summit for less than $50.
Back to diffs - MKIII differentials benefited by adding a Ford friction modifier to the gear oil, but - as mentioned - the MKIV differential is a torsen type and needs no modifers. I don't have scientific evidence that supports my recommendation to use Mobile One synthetic gear oil in your diff. But, I would mention these three things:
1- Almost everyone I've talked with recommends Mobile One. On the other hand, Phil runs a very strong single turbo MKIV and has had good luck with Redline in his diff.
2- Mobile One has decades of thorough testing to back up their excellent products. I bought an RSP from a retired F1 racer who's father worked for Mobile Oil for 30+ years. He has sworn by Mobile One for his professional and personal race cars (he owns a Boxer and a Modena 360 now and an NSX - we'll forgive him for the Acura...).
3- Many auto tranny's have been damaged or ruined due to the use of Redline products. Obviously, differentials are not the same as transmissions, i.e. they don't build up heat like trannies do, but my gut says to avoid Redline.
Now you get to decide.