I am no expert on lefty's but I imagine if it is only a small amount of oil around the filter then it shouldn't be a problem, if you look at the servicing tech note pdf for the lefty it says lightly oil the airfilter so there should be a slight amount of oil in the filter from when it was assembled (hopefully)
And since re-oiling my lefty airfilter I now have oil residue around the holes of the filter boot, perhaps I have over oiled the filter?? maybe yours was over oiled by Cannondale??
Yes, it's quite normal. There has to be some oil on the air filter, at least that's what the manual says. When I took mine off for service, I put on too much and now the damn thing is everywhere So just don't worry and ride it.
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