I've used synth in all my used cars since blowing reverse in my mkIII golf. Synth is supposed to help spool up the synchros for smoother shifting. The old golf's at least shift a bit hard, especially when cold, and it seemed to help. Also put it in my RAV4 at 210k and it really helped. As far as extended life...? You probably will get more just from not grinding as much.
Thats my 2c