Running Rich?

I posted a pic of my exhaust on DSM Tuners cause someone was wondering how a dual tip would look. Got a reply back about how it looks like i run rich. Now that i think about it, I do. There is a nice little black spot on the floor in my garage right behind my muffler. Is this normal with running no cat? If not some possible causes? I have always known it burned oil if i let it idle for 5 mins or more...and since its winter i have been doing that so it can warm up. Think that would be the cause?