If you pack your universal joint and then the next year you pump it full of more grease, how much will get through the 4th main or down the drive shaft? Where does it all go?
On my '24, some oozes out the U-joint ball on every drive. However, I'm sure that some also works its way down the drive shaft housing, where it causes no harm. It would take a lot of years of grease pumping to fill up that housing!
As long as the grease you use does not contain graphite or polymdnym It will mix with the engine oil and not cause a problem. However if it contains the above, it could cause a problem with the magneto.
I would not worry about graphite in this location. The engine oil washes the grease out to the rear and the amount of graphite that would get pulled in, if any, is very small. You are going to get more metal slough from the rings scraping up and down the cylinders (the black goo you see on the magnets etc).
I would use a wheel bearing type grease if you have it. The general use type grease seems a little weeper in the first place and mixed with any hot oil from engine plus rotation of U-joint just wants to fling it out any where it can. Nice big glop's!
Dave, the fourth main is ahead of the universal joint. For excess grease to go down the driveshaft it has to go through the driveshaft bushing.
Hope this helps.
Allan from down under.