I've had similar experiences with synthetic oils in transmissions. They would
work fine in some, and not well in others. Huge improvements in the old
Ford rockcrusher, improvements in the RX-7 (though it destroyed the limited
slip clutch pack in the differential), no change to slightly worse in the
Honda,
much worse in the Toyota. I would speculate that it had to do with the
materials
used in the synchro rings themselves. This would also concern me with the
overdrive, since the engagement surfaces are similar to limited slip clutch
packs.
The corrision isn't simply limited to synthetics. There are different agents in
the GL4 and GL5. You do need to follow the oem recommendations on this
point. I think (not positive) that it's the GL5, with it's hypoid gear
additives
that can corrode some of the brass or bronze components in a transmission
designed for GL4 lubricants. Sulfur is the culprit me thinks.
If you're willing to risk your transmission, you might want to experiment
with Dextron III ATF. It's a very good lubricant. Manufacturers have
realized that transmissions and even differentials do not need the heavy
lubricants that have been used for decades. That's why there has been
an industry wide transition to lighter gear lubes. Going lighter gives
cleaner emissions, more performance and better gas mileage, and the
transmission still lives. Many manufacturers now specify 10-30 motor
oil or ATF for manual gearboxes. It's been a hot rod trick in the
japanese car circles to switch older gearboxes to ATF for some time
now. I personally haven't been willing to do it in my vehicles because
ATF is so thin. My brain can acknowledge the wisdom of the switch,
but my tummy gets butterflies at the notion.
>>> "M. E. Foster" <foster_m@swbell.net> 08/30 9:36 PM >>>
Some synthetic oils appear to contain additives that are corrosive to parts made
from certain types of metals. I wish I could quote the exact source of this
information, but it has been a while since I stumbled across it. One of the more
informative web sites on the subject of synthetic motor oil is
www.redlineoil.com. They go out of their way to point out that their
transmission lubricant MT-90 is not corrosive. I tried MT-90 in a new Miata last
year and I didn't like the result. It produced noticeable gear clash when
shifting from 1st to 2nd and 3rd to 4th when the transmission was at operating
temperature. I assume it was the result of the synchronizers not grabbing
because of the slipperiness of the oil. I changed to Mobil 1 and got the same
results. After putting up with it for a week or so, I changed again to Valvoline
semi-synthetic 80W90 and the problem went away. I just bought a '76 Spit with OD
transmission and I need to change all of its bodily fluids. I think I'm going to
stick with non-synthetic oil in the transmission and differential.
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