Basically, I'm pretty sure I've got a failing fifth gear synchronizer in my manual transmission ( 5th only engages between about 65 - 72 k/h ) and was wondering if something like Lucas Transmission Fix can provide a temporary solution until I'm ready to get the syncro replaced?
The synchronizers need a specific amount of friction in the transmission fluid. If the transmission fluid is too slippery (more friction reducers) then the synchronizers will get consumed. If the transmission fluid has not enough friction reducers the gear wheels can get consumed, especially if the transmission would be a transaxle (differential and gears in one package).
I doubt that with some additive you would be able to exactly hit the needed "slipiness".
For the first try i would change the transmission fluid. Please consult your manual: Should it state that you need an GL4 type of transmission fluid do not put in anything other than an GL4. No GL5, no GL4+ and no GL4/5. If you can't find the indication for the needed transmission fluid you could consult the "oil-finder" homepage of castrol.
Explanation: GL5, GL4+ or GL4/5 have a much higher level of friction reducers than an normal GL4, thereby damaging synchronizers designed for GL4.