Recently purchased a '78 R100/7 and shelled out the tranny in the first 500 miles. Are gearboxes a weak link on these old bikes? Someone suggested that sitting around trashes out the bearings and failure is common. Someone else has informed me that synthetic lube doesn't transfer the heat off the gears well and they overheat. Good source of used parts? Will another used tranny do the same? Any good rebuilt boxes out there? Any help would be appreciated trying to understand this thing.