Originally Posted by ragtoplvr
Bugs end their lives on the fork tube, and some bug residue gets past the seals and dissolves into the oil. There will be some water too.The bacteria eat it. Bacterial farts are pretty nasty.
That's the one.
And just a comment, there's the lazy way to change fork oil - and if it' ain't smelling like sewage it saves a LOT of time. (If it's sewage you need to strip the forks down and clean them)
Just take the caps off, drain what you can ( use a transfer pump from the top if need be) and top up to the correct level with fresh oil, but do it OFTEN. Takes about 5 minutes compared with several hours needed to strip the forks down properly - well worth $10 spent on extra oil.