my quess: you have an oil leak somewhere and the oil is getting on exhaust or other hot part of engine and burning off. Just had this problem with my car. I had multiple leaks. some were visible others were not. I replaced valve cover gasket seal, spark plug seals, end cam seal. also my oil pan gasket was leaking and oil was getting on top of the header and burning off. couldnt see it until I jacked up car and got under though.

See if you can find out what is leaking, if any thing, go from there.