Transmission oil

[Bored&Stroked]

Am I correct in assuming that you used the Red Line 75W140 NS GL-5 rather than the MT-90 75W90 GL-4 simply because you like the higher viscosity than is available in their Gl-4 products? (Red Line states that the NS GL-5 formulation can be used in vintage syncro transmissions.) According to Red Line, the MT-90 GL-4 is specifically intended for brass synchro transmissions.

I wanted a little higher viscosity - and as much shock/load proofing as I could get. Now that might be completely ridiculous (given the low HP levels of even the baddest flathead), but I figured I'd give it a try. Given this is the NS version, it should work out just fine.

I would probably not be running a synthetic oil on a stock 39 trans - as it would probably leak onto the floor at a high rate and is expensive to boot (that was my last experience!). BUT - because of the way I made the cluster and reverse shafts (with sealed caps on the ends) - and the way I sealed up the "shaft pin", it should not leak (and hasn't yet) - so why not try it.

Once it gets warmer, I'll be driving the car again and report back on shift quality during more extended usage. In my testing a couple weeks ago (low 40's temp - only about 10 miles of road testing) - the transmission shifted really well and didn't slip out of gear on high-revs and letting off the gas in 2nd (though this is due to how the trans is built - not the oil). So - I'm optimistic in general . . .

Note: They also mention that you can mix this with the 75W90 NS or MT-90 to "tune" the viscosity.

It would be interesting to see how well this works in a "regular" 39 box? I've not tried it . . . might leak like hell!

 

[Kahuna]

I use the 85-90 Stalube in the steering gear, transmission, OD & rear axle. Seems to work fine.
I figure all the new oils are far superior to what was available years ago

 

[Ross F-1]

...
I would probably not be running a synthetic oil on a stock 39 trans - as it would probably leak onto the floor at a high rate and is expensive to boot (that was my last experience!). ...

It's surprising how little perceived difference there is in the pouring characteristics of StaLube 140 and the Mobil1 85w90. My trans didn't leak a lick more with Synthetic. Just the usual weeping around the ends of the two shafts, a little at the rear (open drive).