Yeah works well in sealed gearboxes! I can see the appeal of it in those applications but with the m620 having a big hole through to the controller I just don't feel as confident about packing it out with it. The excess may get into places I'd rather it didn't. My experience with the SHC100 is it tends to stay where you put it!
The source of the noise might be not only the lack of grease. Inspect the middle gear bearing (the overrun one way bearing) pressed into the larger one gear. The large one middle gear can slide down on that bearing causing the gears rubbing each other making noise (in some cases extremely loud noise). The problem with the middle gear is the helical gear (not straight cut gear) and it causes lateral force under load and the gear can slide on the bearing over time. My 3000W Innotrace X1 controller was able to slide that gear on the pressed bearing.
Helical gears suck, straight cut gears are the best for heavy duty and reliable applications. The only advantage of helical gear is less noise comparing with straight cut gears, but helical gears require specific design solutions to deal with lateral forces (and Bafang did not implement such solutions so those gears can fail).
