I had my 4 speed trans rebuilt about 9 months ago by an SF Bay Area expert,
but when they put it back in, it crunched when shfting fast from 1-2 and
2-3, especially noticeable climbing hills. A couple of weeks ago I took it
back and they pulled the trans, changed synchro rings, and put it back in.
Now the 2-3 crunch is fixed but it still crunches when speed shifting 1-2.

I'm going to bring it back in again. But I want to have some information on
why it would do this. I was under the impression that if you change the
synchro rings, it stops crunching. But apparently there's a conflict between
moly synchro rings and Redline synth oil, or some such.

Clutch plate, hydraulics, etc. are excellent.

Any transmission people care to comment?

Again, these are reputable rebuilders. I'm not out for a vendetta, just
trying to understand the problem better to get it fixed. Other than that,
the shifting is great. What's going on?