I fitted a nipple to my Dr. Zed when I got it and work on the theory that if I pump grease in until the black, mucky, emulsified crap is replaced with clean grease, it must be a good thing.
If you do a lot of hard trail riding in all weathers then water and muck will get in. Add to that some dropping in rivers and jet washing. Also, as mentioned, the oil in the frame does warm the grease enough for it to seep out past the lower seal. I'd rather give it a few pumps after every ride than strip it out. A lb of grease lasts a long time and only costs a few quid.
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