Some time ago, someone started a discussion on the list regarding the
type of transmission oil that should (or perhaps I should say may) be
used on the Moss box. There were a lot of interesting comments
regarding the weight of oil and whether it should be straight motor oil
or gear oil. Since then, I've done some research and a few experiments
with my +4 that may be of interest to Moss box users. My main concern
was to determine which oil was best at promoting smooth shifting,
reducing gear noise, and keeping the transmission cool. So I tested
different oils over a period of time to deduce, purely subjectively,
which type/brand of oil I should use. I tried 90 weight GL5 gear oil,
30 weight motor oil, 75-90 GL5 synthetic gear oil, 75-80 GL4 synthetic
gear oil, and 75-90 GL4 synthetic gear oil. Before I give you the
results, let me tell you what I found about motor oil vs gear oil
weights and the GL nomenclature. Apparently, motor oils and gear oils
are tested at different temperatures to determine their weights (or
viscosity). For this reason, a 30 wt motor oil is approximately as
thick as an 80 wt gear oil, while a 40 wt motor oil is roughly
equivalent to a 90 wt gear oil. The GL designation is used to determine
the kind of service for which the oil was designed and the kind of
additives mixed with the oil. The most common major brands of gear oils
now days go up to GL5 and can be used where "gears are subject to the
high pressure of hypoid designs…" Some manufacturers claim that GL5
oils use additives that are not healthy for brass synchronizers and may
be too slippery for manual transmissions (and the Moss box does not have
hypoid gears). These additives supposedly eat up your synchros twice as
fast as those used in GL4 oil, but I'm not willing to run a test to
verify this. Anyway, here is what I found. Remember that results are
subjective and purely unscientific. Smooth shifting was judged by feel
and crunchiness, noise by what I heard, and temperature by feeling the
transmission after running the car for about one-half hour.
90 wt GL5 Gear Oil - This is what I was running when I started the
"experiment" and I was not pleased with it. Shifting between 1st and
2nd gears was impossible without crunching until the transmission warmed
up, and it usually took 15 to 20 minutes. Once the tranny was
thoroughly warm, shifting was OK but not very smooth. Heat was not a
problem and neither was noise.
75-90 GL5 Synthetic Gear Oil - Shifting between 1st and 2nd was still a
problem when cold, but not as much as with straight 90 wt. Shifting
also smoothed out faster than with 90 wt. Heat and noise were about the
same as before.
30 wt Motor Oil - This change made a major difference. The shifting
problem when cold disappeared completely and shifting overall became
noticeably smoother. But I did notice that the transmission ran hotter
and slight whining noises that were not previously there became
noticeable in 2nd and 3rd gear.
75-80 GL4 Synthetic Gear Oil- There was little difference between this
one and straight 30 wt motor oil. Perhaps this can be expected since
their viscosity is very similar. Shifting was a little smoother, but
not by much. Temperature and noise were about the same.
75-90 GL4 Synthetic Gear Oil - This one was somewhere in between the
75-90 GL5 oil and the 75-80 GL4 synthetic. Shifting between 1st and 2nd
was not as smooth through all temperature ranges while the noises
associated with 30 wt and the 75-80 GL4 synthetic oil went away. The
tranny seemed to remain about as cool as with the 90 wt GL5 gear oil.
Keep in mind that I ran all these tests in California, where winters are
mild and summers are cool (SF Bay Area). And, as I said before, the
results are very subjective and cannot be backed up by pure hard facts.
But I did change the oil I use to 75-80 GL4 Synthetic and I am quite
pleased. Shifting when cold is nice and smooth and it remains so
through all temperature ranges. The tranny is also reasonably quiet,
but not as quiet as with the straight 90 oil or the 75-90 synthetics. I
still have to double clutch when down-shifting, and I don't think there
is a magic transmission oil out there that will tame this particular
"charm" of the Moss box. I chose the synthetics over straight 30 wt
engine oil because of the possibility that the additives in a gear oil
may be better for the transmission than those found in engine oil. What
was the best part of the test? Driving the Mog, of course!
Armando Picciotto
'63 +4
|