Left front? Have a look at the crank position sensor, on the left front. It's possible you did something to it when changing the filter. I believe there's a warning in the shop manual about that possibility. Hope that's not the case for you...

 

Just looked up your profile. Your bike is a '06, right? Take it to the dealer. Make sure, before you do this, that all your fluids are level. It will be fixed under warranty. No need to tell them you over filled it, just take it in. IMHO

 

Is it possible that one of your forks is leaking oil? That happened to me with my old Fat Boy. The cap at the top of the fork was loose and leaking. Oil was runinng down the fork and splattering onto the engine. It wasn't a lot, just some spots here and there. Another possibility is the head bearing grease is leaking from the neck. Do you see any oil forks?