What would cause my Craftsman tractor to smoke excessively?

I recently purchased (at a give away price) a Craftsman lawn tractor model 917270711 with a 17hp Briggs engine at a yard sale. The tractor has been garage kept and visually appears to be in excellent condition. The lady said her husband couldn't get it started got mad and bought a new mower. The battery was dead so I installed a new one and the mower fired right up and ran great. Shortly thereafter it started smoking so bad that I had to turn it off. Upon further inspection I found the oil to be as think as molasses with a dark green tint like 2 cycle oil. It also had oil dripping from the muffler, air filter and there was oil around the outside of the carb/air filter housing. I cleaned everything up, sprayed out the muffler with carb cleaner, changed the oil and fired it back up. The smoke slowly disappeared and the motor seems to run really good however I have noticed that I still have heavy blow by coming from the air cleaner housing. The local lawn mower guy says that the mower may have been tilted excessively allowing oil to run into the exhaust which is the reason for the heavy smoke. I have less than 1 hour on the engine since I changed the oil and I'm not sure if this theory is correct. I purchased this lawnmower for my son as a gift and I don't want to give him something that has problems. Even though the engine starts and runs great I have a gut feeling that there are internal problems. Any suggestions or should I just let it run and see if the problem goes away?