Well, yes and no.
Synchros, like limited slip diffs have to be able to apply friction to the mating surfaces to work. The slipperier the oil the less effective they are. Imagine your brakes covered in various different grades of oil, same thing happens with the synchros,they are in effect a 'brake'. The trick is finding an oil that has the right balance of friction and lubrication. The oil filled in his 100,000km service may have lubed the gears and bearings mint, but the synchros couldn't slow the rotating mass of the box sufficiently with that particular oil, doesn't necessarily mean they're rooted.