I've not had a problem with the Eibach poly bushes and supplied grease on my M3 sway bars, three years now and counting I think. Are you taking all of the preload out of the bars before you tighten the bush mountings? Both wheels on the same axle need to be at ride height when you tighten them up... you really need ramps or a drive on lift.
Anyway, the answer to your question is heavy duty marine grease. ?