Lower viscosity ratings will improve shifting.

 

I spoke to a tech at Castrol today because I was trying to find out if I could get some friction enhancing additive to help soothe the shift into third. Since that wasn't possible I asked about using a thicker oil, like 75W-140. The reply was "no, since that means the oil will take even longer to get out of the way and leave the synchro cones time to work".

 

Conventional wisdom would otherwise lead me to believe that a thick oil would tend to have more drag and hence enable a better synchronization. He went on "well, the oil would stay between the cones and the thick film would provide fantasic lubrication". But no speed matching.

 

So, I will be draining my much loved TAF-X in favour of some SMX-S. The ratings are 75W-90 and 75W-85.

 

I'll drop back in when I have tried it out

I've got 80w90 swepco in mine now. Worked better then the 75w90 Neo Synthetic.

 

I think the older boxes like the thicker oil....or at least mine does. It's still not perfect....but better.

Wouldn't you think that those two oils have the same viscosity at operating temp?

 

Maybe it's just the additives making the difference? I am gonna risk another bundle of money on the 75W-85. If that don't work, I'll switch back to EPX 80W-90. Based on the fact that it's cheap!

The additive package I'm sure is different. You could tell just pouring the two that the NEO was thinner then the Swepco....who knows

Setright, I have excellent shifting with the Redline MT-90, even with that gumby shifter bushing still in place. The synchros don't hook instantly, but they're a heck of a lot quicker & lighter than they were with whatever was in the tranny when I bought the car. Previously I had to use force the get lever into another gear.

 

The only question is it's GL-4 rating and whether or not the differential gearset suffers because of it.

Yeah, that GL4 thing bugs me too. Otherwise I would tried some other oils before. I have just foound that Motul makes a 75W-80 API-GL5 gear oil. Trying to get some now...

I forgot to mention that my limited experience bears out that the thicker gear oil does tend to get in between the synchros and slow the shifting process ...more so when the cold weather sets in. It doesn't seem to matter a whole lot in hot weather.

 

Years back I experimented with a 50/50 mix of Mobil 1 75W90 & Castrol Syntec 5W-50 in my Colt tranny. It was a just stupid hunch, but it worked really well. I changed it frequently tho because I figured the motor oil was probably shearing.