yeah see I'm really pleased with my synchromesh, I had a 3rd gear grind in my junkyard trans, it's pretty well gone unless you absolutely SLAM into 3rd gear.
that makes sense about gear oil it's meant for older stuff, diffs, older trans like in my buddy's 77 camaro, oh that's a beast of a car.
if you run motor oil in the trans change it about every 12k miles like the haynes manual recommends, I change mine with all the city driving I do about every 10k miles. course I like to drive mine hard anyway so I take care of it so it takes care of me and don't dump me on the side of the road ha ha.
up to you what you use, synchromesh is worth the $$$ though, motor oil will work cheaper.