Dailydriver tried RP synchromax a number of years ago in a stock 2000 T56 with the OEM cellulose lined blocker rings. He had immediate problems with grinding shifts, worst that he ever had with any fluid. Dumped it 24hrs later for a synthetic ATF and it was fine.
If you have the 2001 and later style carbon blockers then Synchromax might work ok. Most that use it seem to be ok with it. But, who knows how many tens of thousands of miles they got on their synchros before selling the car or how long they would have lasted vs. some other ATF or MTF.
Here's a post on the use of varying fluids in a T56. No easy answers and lots of opinions. The research I did on it was summarized on the last couple of pages. What fluid should I use in my T-56 as per Tremec techincal support Choosing a manual transmission oil for T56 and others