Small Engines (Lawn Mowers, etc.): White smoke and oil coming out of muffler but sounds ok, sponge filter, weight oil

Question
I have a 3.75 B&S motor on a 22" deck that started pouring out white smoke one day while mowing.  I also noticed that it was emitting some oil out of the muffler as well.  During all this the motor sounds and appears to run good.  I emptied the gas out and changed the oil and made sure I didn't overfill the oil.  I removed the plug and gave it a few pulls to get oil out of the chamber.  The spark plug had a lot of oil on it so I replaced it with another.  The sponge filter was soaked in oil as well.  I cleaned the filter and re-assembled everything.  Now I can't get it to start and it's still emitting oil out of the muffler.  Before I did all this the mower started with ease and ran good besides the white smoke and oil coming from the muffler.  Whats wrong?

Answer
Are you using SAE-30 weight oil?  Have you tried cleaning the valve breather.  It is located on the muffler side of the engine and retained by 2 screws.  You can clean the valve breather with parts cleaner or carburetor cleaner.

Obviously there is oil in the muffler and cylinder that is causing the white smoke.  You can clean the cylinder and muffler with carburetor cleaner.  I would remove the muffler to clean and ensure it is completely dry before reinstalling.  If you remove the spark plug and spray carb cleaner in the cylinder and pull the string you can remove most of the oil in the cylinder.
How old is the engine?
Let me know.
Eric