CLightning

It has been found to be true in many cases that switching from dino to synthetic motor oil on cars with old seals and gaskets have introduced new leaks. (This already happened to me and I'm still procrastinating borrowing an engine lift to reseal the pans with FIPG)

The question is: does this also apply to using synthetic gear oil in high mileage manual transmisions and differentials as well?

I'd like to know before spending $$$ on Redline MT90 that many people here swear by, As of right now, I have zero manual transmission and differential leaks. My SC300 has 230K miles and I don't think those fluids have ever been changed. The last thing I want is more leaks.

Biddles

iTrader: (2)

At that mileage I would stick to OEM Toyota fluid. Not worth the risk of replacing seals, gaskets, etc... If your car had less mileage on it maybe, but at 230K if it's not broken don't fix it.

KahnBB6

iTrader: (5)

CLightning, if your car has 230K and the fluids haven't been changed, then change ALL of them! I highly doubt that's the case but if so that would be ridiculous negligence by the previous owner(s) before you.

I do, however, recommend Redline MT-90 for W58 and R154 transmissions. My SC has 235K on it and I've run that type in both my original W58 and my current R154. I'm not aware of this fluid causing leaks. It's simply nicer on the gears and improves shift quality. It is also the ONLY type of oil that should be used in all R154 gearboxes to keep them the happiest.

Now your rear differential fluid (stock open diff or even a swapped-in Torsen LSD) does not need to be anything terribly special. A simply Mobil1 synthetic gear in the same oil spec listed in the SC300 owner's manual is all that's needed, though Redline oil can be used there too (without any friction modifiers though).

You should not have leaks simply because you're using a better transmission oil than what the manual specifies.

Aside, I should note that GM Synchromesh used to be a second best alternative for better transmission oils but I think MT-90 slowly proved to be the better of the two among Supra owners, which it's why it's the one everyone tends to stick with and recommend.

CatManD3W

iTrader: (18)

I use both Redline MT-90 in all my vehicles for transmission and Redline 75w90 in all diffs...

I run this both in my SC and in my Tacoma.....Never had any issues with them at all.....it makes everything smooth as butter

My SC had 170k on original w58 and rear diff.....I switched them out to the above redline products....

I do this on every vehicle I buy....because a lot of people neglect every changing those fluids.....