Riding Lawn Mowers

I have a YTH 22v46 tractor that I purchased in May 2014. It has 18 hours on the meter. For the past 10 hours the tractor has become increasingly harder to start, though when it starts, it runs perfectly fine. When it starts, it spews any oily residue out the front (muffler) onto the concrete. (staining concrete with oil). Any ideas as to what is going on here. Air filter is clean and gas is fresh. Plugs look normal.