Well best method is to put a piece of tissue paper inside the finger of a rubber glove, then attach the finger over the end of the breather pipe connection, wrap an elastic band over to secure it... Use a second finger to cover and block the inlet on the air intake pipe.
Now take the car for a drive, making sure it is warmed up and at all revs.
When you get back, remove the finger and check how much oil is inside... It should just have a light coating on the tissue but if its soaked in such a short time of even half way soaked, then you have a possible cause.
If it's a 99 model then it shouldn't be fly by wire so it is releatively safe to touch and move the throttle butterfly by hand in order to get it clean, You can confirm this by checking to see if there is a throttle cable to the body itself... If it is CF2 Fly by wire... follow the earlier posted proceedure instead.
BTW, if it's not fly by wire. A throttle reset proceedure won't actually make any difference mate
Also check this from one of our members "Gazza 82" who has had similar issues... an interesting read
look at the "idle saga" link