OIL Leak

I have a 99 intrigue with 3.5 and I have about 105000 miles. I just bought the car 5000 miles ago. I use amsoil 10w30 synthetic and am using the same amount of oil as most of you all are, about a quart every 1500 miles. I changed the oil today and I switched to mobile 1 5w30 synthetic. Mainly because I didn't have any 5w30 amsoil and I am curious about gas mileage. I only get around 26 with 80% highway miles and wondering if the oil will make a differance. Anyway I noticed oil on a round cylinder underneath the car directly in front of the oil drain plug. It is about the size of a 20oz pop and looks like it is a part of something bigger. I couldn't get a real good look at it. Any ideas on what could be leaking oil on that side of the car? I will post back if my gas mileage changes with the change to mobil 1