this is rubbish
i ran 75w90 motul 300v LS in my EK9 as it was required due to the spoon plated LSD, there was never a single issue with selecting gear, i ran it through scottish winters like this too with my spa oil gauge showing the engine oil in negative figures on start up (also ran it at the other end of the scale covering ALOT of track mileage again with no issues)
grinding synchros is due to wear, simple, a decent oil will purely mask the problem. honda synchros sadly are very delicate and prone to wear, i have rebuilt numerous B series gearboxes and they have all shown a decent amount of wear regardless of mileage
helical diffs will also not make any clunking, a light hum if anything! the noise on a plated diff is purely down to the clutch plates rattling about when there is insufficient torque applied