Oil Leak

I have a Briggs & Stratton 7.75H.P. engine on my Craftsman snow thrower that is leaking oil. Engine model number is 12C314-0131-E1. Engine is 15 years old but still runs like a top. Oil is pooling at the front lower corner where engine is mounted to frame. I cleaned the engine up thoroughly to try to pinpoint. I originally thought that the front PTO oil seal was the culprit but that is not so. The definite area is the area that I originally mentioned. Could it be the rear oil seal (flywheel side) which I cannot see? I am pretty handy as I changed both belts and idler wheel yesterday and as you all know that is not the most easy job...splitting the snow thrower to change the belts and taking the drive apart to change the idler wheel. Hope I did not waste my money. That would have been the time to pull the engine to take a closer look but did not want to go through all that trouble before asking you all. Maybe it is time for a new snow thrower??? Thank you all for any help that you can provide.

With that age on the engine, my first though would be the sump cover gasket leaking. they get brittle over time and will crack couples with the loss of torque on the cover bolts. and part of the gasket will becomes sucked into the engine causing a gap and hence the oil leak.

Thanks for your reply Ilengine. The engine is 15 years old. I have eliminated the following...it is not the crankcase gasket. it is not the front PTO seal. It is not the drain plug(s). It is not the filler tube. The only things I have not checked yet because of time are the valve cover gasket or the rear seal behind the flywheel. You mention the sump gasket. Where is the sump over gasket located? Is that the same as the crankcase gasket? The only other thing could be a crack in the bottom of the crankcase.

Purchased front and rear seals plus crankcase gasket. I had the day off yesterday. Drained gas and oil then pulled the engine off. First thing I checked was for a cracked block underneath. No crack thank goodness. Really inspected engine and could find no oil leak evidence anywhere. Pulled the crankcase cover and changed the seal. No evidence of oil leak. I was surprised how super clean everything inside the crankcase was. Probably because I change the oil every year. Proceeded to rear. Flywheel came off pretty easy. Changed that seal. No evidence of oil leak. Really took a close look at rocker cover area. No evidence of oil leak but am going to check and adjust (if needed)valve lash since I bought a new gasket for that too. Rear and both side drain plugs show no evidence of oil leaks. I am stumped. I still have the engine on the bench in semi-torn down state. Before I put the crankcase cover back on is there anything else you can think of before I put it back together? Could it be oil from when I tilt the thrower forward and set it up on the auger housing that oil leaked over the years and built up on everything? But where would that oil be coming from? I tilt it up at least once every season. Now I feel like I cried wolf for nothing! It just bugged me as the thing didn't leak oil for at least ten years. Thanks!