At an idle the oil has the time to sit in the head and seep past a worn valve guide/seal into th combustion chamber. Under power the oil is pumping through the head so fast it won't seep past the seals.
Do a leak down test to determine if you're losing compression, if you are, rebuild the top end.

 

If it's guide seals it goes like this.
At idle the throttle plate is closed almost all the way. This puts heavy vacuum on the guide/seal. The engine is trying to get air anyway it can. Through the guide is one way and it takes oil with it. At speed, the throttle plate is open more and there is not so much vacuum in the engine so it tries to get air through the carb instead of the valve guide.

Sorry, the engine is going to need work. They just don't heal themselfs. Sorry, but this can get into $$$'s.