The gears in a SM 420 are big, heavy, and have a lot of inertia. A lubricant as slippery as synthetic gear lube is probably keeping the synchronizers from doing their job properly. Rice rockets with transmissions that would fit in a back pocket and gears that weigh a fraction of what's in a real transmission don't have that kind of inertia to deal with. Some people also think the new water-thin synthetic motor oils are OK to run in stovebolt engines- - - -WRONG!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!