I have a lawn tractor with a 16 hp Briggs engine. It burns oil. I put Lucas oil in and it helped. However the problem is back! There is oil showing up in the air filter. Obviously the engine smokes. What are the possible causes?

You have a faulty breather assembly, it is a small rectangular shaped device that mounts on the side of the engine, and has a "breather" tube going from it to the air filter chamber. This is a simple fix. Be sure you have the new gasket with the new breather assembly. Carry your engines model, type or code, and serial #s to a local shop and purchase the breather assembly.