This is a timely post.
I just (and I mean Tuesday afternoon) got the speedster back on the road with the 2110 meant for my bus, and a transaxle rebuilt by @Anthony in Kalifornia.
I put $11/qt Mobil 1 75/90 GL5 in it, as I always ran a cheap semi-sythetic GL5 and never had any issues. I could barely shift the thing when I rook it out for a spin (double clutching, etc.). A quick call to Anthony confirmed that he wanted GL4 (which I suspected). Apparently Brad Penn makes some GL4, but I can't buy it here.
All I wanted was some GL3 gear-oil like I used to crank in by the half-gallon, back in the day at Bruner Bros. Oil. In 2020, out here in flyover country-- finding something besides GL5 was like looking for shards of the true cross. None of the FLAPS had it (and I checked them all)-- everything was GL5.
I had a couple quarts of Swepco 201 in the stash, but it's GL5 as well, and I didn't want to drop another 2.5 qts of super-oil with the same result. Advance, Autozone, and NAPA had some $10/qt GM syncromesh GL3 lube of indeterminate weight, which I nearly bought-- but thought I'd try a speed shop as a last resort.
Winner's Circle Racing in Peoria had some Redline full synthetic GL4 75/90 for $17/qt-- which is pretty harsh for a break-in oil, but it is what it is. I just came back from a little 20 mi shake-down. The syncros are still pretty tight, but they loosened markedly from the start of the drive to the finish. Rev-matching helps considerably, and if it doesn't want to go into first, I just double-clutch. It'll be great as it loosens. I moved to a CB rhino nose-cone mount (from a Prothane)-- I was hoping it might be a bit quieter, but alas, no. This also is par for the course.
I suppose the snooty gear oil only cost an extra $30 for the RedLine (over something more generic) every time I change it, but it still sticks in my craw, as I'd already dropped $35 for the Mobil 1. I'm nearly $100 into gear-oil, and I've only driven 50 mi. We used to just about give this stuff away, way back when.
I'd love to know what cheap and readily available oil might work in a syncromesh transaxle for break-in. I can tell you with 100% accuracy that it's not Mobil 1.