I would recommend you not to use any synthetics or LSD oils with very low coefficents of friction in a Torsen style diff. It tends to reduce the effectiveness. For the diff to work, it needs to load the thrust surface of the gears and face of the worm gears to the case to lock both axles together. Super slippery oil won't let it do that. Any high quality GL-5 rated 70-90w oil will work fine in the Supra diff for almost any application. As long as its clean, it will work well. Just change it more often for extreme applications.
Several people I have worked with used synthetics and would notice the rear diff would slip. They thought the diff was bad and were ready to replace it. I changed the oil and it worked great again.
I am not sure about the rear diff of the mkiv, but if one of the experienced guys is saying there is a mag tip, then there must be. The thing is, some manufacturers put a magnetic tip on the drain plug to catch all the metal bits so you can inspect the metal content of the fluid while it is draining. Helps you figure how much the engine is eating the metal parts.
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.
...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.
Actually, I haven't heard of a single auto tranny damaged by Redline. On the other hand, has anyone heard of successful use of Mobile 1 in either auto or 6spd mkiv trannys? Redline did swell a few bushings in 6spd's, but it's not like the tranny was completely ruined - the shifter didn't self-center anymore.
Either way, I never recommended Redline for the tranny. Oem V160 in the tranny, and Redline Heavy Shockproof in the diff.
Royal Purple #9 may prove to be an acceptable substitute in the 6spd, but I believe that the jury's still out...
Phil - I think we're snagged on a few words, and there is plainly a misunderstang here. You did not ever recommended RedLine for the transmission, and I did not ever recommend Mobile One for the transmission. You and I both agree with the clear fact that the only fluid for the automatic transmission is the Toyota factory ATF, period. Again - from both Phil and me - DO NOT USE MOBILE ONE FLUID IN YOUR TRANNY !
We simply have different recommendations on which fluid is best for the differential. I prefer Mobile One gear oil for this application. You've had good luck with Redline. Both are valid opinions to be mutually respected.
A few weeks ago, my friend Matt did a poll (if I had time I'd search for and attach the thread) in which several people attributed Redline in the tranny as the contributor/cause of damage to their tranny. Because of this information related to transmissions, I choose to avoid RedLine altogether. I admit that this may not be completely scientific, but it is nonetheless part of my reason for suggesting Mobile One - for the differential.
Another reason for my recommendation of Mobile One in the diff is the excellent track record of their gear oil, and the related success of their synthetic engine oil that many, if not most, of us use in our MKIV's use.
On a rather ironic note, I bought my RSP in April with RedLine in the diff, and have carted two quarts of Mobile One around with me since then and never have taken the time to swap it out. My diff is working just fine.
No prob, John. I should have been more clear as well - we both have no idea how good (or bad) it would be to try Mobile 1 in the tranny. I'd suggest that it may very well have produced negative results similar to using Redline in the tranny. Therefore, I'd suggest that the way Redline behaves in the tranny has very little to do with how it performs in the diff (just as our lack of knowledge regarding Mobile 1's performance in the tranny should have nothing to do with whether it is recommended for the diff).
I also agree that both Mobile One and Redline have excellent track records for both their gear oil and their engine oil. As you probably know, both of these brands are commonly used in MKIV's.
P.S. I'm only 7 posts away from 'Senior Membership'
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