I have a 1996 16hp Briggs and Stratton, horizontal shaft motor on a reel style riding mower. When I started it this season and ran it for some time and it started to blow blue and white clouds of smoke randomly. I am wondering if it burning oil or bad gas or something else? I do not want to run it if is going to cause more damage. Any suggestions?

That sounds like oil burning, whens the last time you changed your oil?

Check and make sure oil level is not overfull. Check air filter for oil contamination. A leaking head gasket can cause engines to feed oil through the breather into the carburetor.

Best of Luck....