I have a 1999 AC 300 that has been acting very weird. When cold the engine will start and run fine, if I drive it for about 5 minutes and it warms up it will a) not idle any more and b) start to belch white smoke out of the exhaust (smells like burnt oil). The white smoke happens and the engine barely runs, and right where the exhaust exits the engine there will be spots of oil. If I let it cool down, the engine will start and run ok for another 5 minutes and it starts all over again.

Any ideas? Here are my thoughts:

1) Bad piston rings (warms up and increases tolerances)
2) bad valve or valve guide?
3) Is there another seal in the engine that would do this (head gasket?)

Thanks for the help