I am no tribologist, so this is just anecdote and opinion, but I put synthetic oil in the 5 speed manual transmission of a Toyota once
I’m not sure about Toyota…but Nissan had somewhat of the same problem…but it had nothing to do with Synthetic oil…
Certain transmissions from Nissan required GL-4. There is a newer oil called GL-5 which they claim is 100% compatible with GL-4…Well it’s NOT. GL-5 eats the Brass Syncro’s. GL-5 contains sulfer…GL-4 does not. So people who put GL-5 in their transmissions ended up rebuilding that transmission about a year later. Later Nissan stated in their owners manual and service manuals to NOT use GL-5…only GL-4…GL-5 WAS allowed in the differentials though.
I suspect that it wasn’t the synthetic oil you used…but the wrong kind of oil.