I bought three quarts of GM Synchromesh, a high-grade manual transmission fluid, from Village Pontiac GMC for $11/quart. I flushed and filled the transaxle, and after taking a quick test drive, noticed an almost immediate smoothing of shifts; rather than a clunky feel, shifting is now rapid and clean. This rates as a definite improvement. I'll finish off the last quart on the transfer case later.
The other oil I was considering was BG Synchroshift, another excellent manual transmission oil (and actually rated as GL-4, unlike GM Synchromesh). However, after seeing plenty of comments from people who've used both and preferred the GM product, and the easier availability of it, I went with the Synchromesh.