[TR] Should I put Redline MT-90 in my Triumph diff?

[Dave MacKay]

On Fri, 29 Jan 2021 Sujit Roy triumphstag at gmail.com asked:



>Anything against putting this stuff in my diff. I have in  my manual
gearbox in my Stag



I looked into this when trying to find an appropriate fluif for my TR3A’s
differential. I don’t know how relevant this would be to your Stag, but
I’ll offer it up anyway.



The contributors to his list showed a preference for:

- Redline GL-5 75W90, or

- Royal Purple Synchromax 75W90, or

- Kendall Gear Oil, 75W



With that in mine, I contacted Red Line Oil, the makers of MT90. They
replied that “The TR3A differential originally called for an SAE 90 GL-4
and the MT-90 would be recommended”. When I asked about the caution on
their web site that “MT-90 are not for use in differentials with hypoid
gears”, Red Line responded that “The note on our GL-4 gear oils refers to
use in a high offset hypoid application which your Triumph doesn’t contain.
The GL-4 gear oils are suitable in zero and low offset ring and pinion
applications.”



The late (and much missed) Randall Young advised that “MT90 has friction
additives” and felt that one would want friction in a transmission but not
in a differential.



When I asked Red Line about the friction additives, they replied that “If
you wanted a more slippery product one of our GL-5 gear oils such as the
75W90 or 75W110 would be options, they contain a friction modifier and are
compatible with phosphor-bronze as they contain a copper deactivator”. Red
Line summed up by saying “In your differential technically either could be
used, the 75W90 contains a friction modifier beneficial overall for
friction and temperature reduction.”



As a result of that investigation, I’m using Redline GL-5 75W90 in my
TR3A’s differential. I use the MT90 in the gearbox. I’ve had no issues and
noticed no ill effects when I examined the diff during a rebuild last year.



However, the Red Line oils are hard to come by where I live, and are quite
expensive (about $25/litre). Some of the other TR3 owners locally have been
using Sta-Lube GL4 85W90 Gear Oil which is about half the price of the Red
Line products and is readily available from NAPA.



Regards,



Dave MacKay

1960 Triumph TR3A, s/n 68639L

near Toronto, Canada
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