[TR] TR3 Gearbox Oil

[Randall]

My previous reply was apparently too long, so here is the abbreviated
version:

> A TR3 Gearbox needs SAE 30 oil.
> A TR4 gearbox needs SAE 90 oil.
> Does anybody knows from what number of gearbox SAE 90 is needed.

The actual change point did not correspond to any change in the gearbox.
The factory switched back and forth several times, at one point saying
either one was OK (without overdrive as I recall).  However, around 1959,
they started specifying only "hypoid" GL4 gear oil.  Supposedly (3rd hand
from an ex-factory engineer) this was because they were having too many
failures under warranty, and felt that the gear oil would provide better
protection for the thrust washers.

The root of the GL4 vs GL5 controversy is a particular additive that is used
on some gear oils (both GL4 and GL5). The additive breaks down in use and
forms weak sulfuric acid, which can over time erode the synchro rings and
thrust washers.  I believe the secret to avoiding this additive is to look
for a gear oil that is also rated MT1. The MT1 specification includes a long
term, high temperature stability test, that the "bad" additive does not
meet.  I ran a GL5, MT1 gear oil in my TR3 differential (which also has
brass thrust washers) for 20 years of daily driving, and no ill effects
observed.  In fact, I credit the GL5 for making it live as long as it did,
since it was already badly worn when it came to me.

However, in my opinion, the absolute best oil for a TR gearbox is Redline
MT-90.  It is considerably more expensive than the other choices, but makes
the synchros work noticeably better than either motor oil or ordinary gear
oil.  Unfortunately it's rather hard to find at the store, so you pretty
much have to mail order it.  I just bought a case on eBay for a bit over
$12/quart with shipping.

-- Randall