Small Engines (Lawn Mowers, etc.): lawn tractor puffing blue smoke

Question
I OWN A TROY BUILT LAWN TRACTOR B.S ENGINE 17.5 HP EVERY ONCE AND WHILE IT WILL PUFF BLUE SMOKE OUT THE MUFFLER.

Answer
Do you notice the engine consuming oil?  If so, how often do you have to add oil and how much oil do you have to add to reach the "full" mark?

Does the oil smell like gasoline?  Is the oil thin and runny?
When was the last time the oil was changed?
What weight oil are you using?
How old is the engine?

Let me know.
Eric