Going to bump an old thread here...
A month or so ago, I had rebuilt a Nissan W71B 5-speed.
Nearly identical to the W58, except it came apart and went back together a lot easier than the W58.
Same style bronze syncros, same size bearings, shafts, shift forks, etc..
Since I had red line MT90 on the shelf, it went in.
Fucking thing shifted like shit !
Drained out the MT90, threw in some Mobil-1 gear lube.
It shifted better, but still not what I would expect out of a fresh rebuild.
Came across a thread on gear lubes & shifting problems that had a bunch of info, but basically saying that the new synthetics or semi-synthetics, although they meet GL-4 requirements, don't get the job done like an old dino lube.
One of the few remaining petroleum gear lubes around is made by CRC, under the Stay-lube name, in SAE85w90, and is sold at NAPA stores.
Picked up some, put it in the trans, and all shifting problems went away.
Guess I would rather change out gear oil once a year, than put up with shifting problems.