GM Synchromesh is supplied by Penzoil and is the same as Penzoil Synchroshift.
Of the various fluids that CEG members have tried and reported on, I don't remember much about Synchromesh or Synchroshift. I suspect that it would benefit from 2 oz of friction modifier. I believe that it is a GL4 gear oil and should be fine.
Here are some of the other products that have been used:
Mercon (factory fill),
Mercon with friction modifier,
Mercon V (with or without friction modifier)(works better with friction modifier)(a semi-synthetic multi viscosity blend similar to ATF+4 except for much less friction modifier),
Mobil 1 ATF (which is a full synthetic Mercon) with friction modifier (also known as the "cocktail"),
ATF+3 which is already very high in friction modifier and needs no additional (works extremely well),
ATF+4 which is the latest spec from Chrysler and mostly replaces ATF+3 (is either a synthetic blend or a full synthetic, Chrysler won't reveal it's composition)
Ford full synthetic MTX honey colored GL4 (factory fill starting in January 2000 as well as factory fill on Focus)(also specified for replacement or top off of earlier MTX75 transmissions that had Mercon for factory fill)(works best with 2 oz of friction modifier)(some have speculated that this is Penzoil synchroshift's more rare synthetic),
RedLine MTL and MT90 (of the two MTL seems to have a more loyal following)(It is reported tha MTL does not need additional friction modifier while MT90 does)(both are full synthetic)(one is more like GL4 gear oil while the other is thinner more like atf but I don't know which one is which).
I personally have used ATF+3 and Ford honey with added friction modifier as well as ordinary Mercon in my own car. I have Ford honey with added friction modifier in it now. I think that it is about the same as or slightly better than ATF+3. ATF+3 is a huge improvement over Mercon. I have three quarts of ATF+4 to try when it is time to change the fluid again in about another 20,000 miles.
Probably more than you wanted to know.
