It's probably burning oil no matter what speed you're at. If oil is getting into the cylinders at a constant rate, then it will look worse at idle because the engine isn't pumping as much air/fuel through it at idle to dilute the burned oil as when you are driving down the road at 2,000-2,500 rpm.
Also, the actual mixing of the exhaust with the outisde air is increased when you are driving too. At idle, you're not moving so the outside air is more stagnant and does not mix and dilute the exhaust smoke as quickly.
You can use some Smoke B Gone or Bardahl's No Smoke to help control oil loss, but definitely keep checking your oil level...running out of oil would be much worse than just smoke out the tail pipe. But eventually, you'll probably want to do either a rebuild or at least replace the valve guide seals.
Good luck!