hard shifting 833

It's not so much the weight of the oil as to what kind or lack of friction modifiers that are in the oil. I used synthetic 90W oil and it didn't work either.

All the gears pairs in the tranny are meshed all the time. The gears on the main shaft are running free on the shaft, the syncro assembly is on splines on the main shaft so they are turning at the speed of the wheels. When you shift the syncro assembly starts to contact the gear being selected and acts like a clutch to speed the gear up or slow it down so the teeth on the syncro engage the teeth on the gear smoothly.

If the oil is too slippery this matching of speeds does not happen and you crash the teeth on the syncro into the mating teeth on the gear. This is were the GL4 vs GL5 rating is important. From what I have read the new GL5 rated oil has additives that do not play nice with Brass syncro parts.