get someone to rev it whilst you make sure its definitely coming out of the exhaust, and not just from under the car being blown back from the engine bay.
if you say you have an oil smell under the bonnet too, it could be an oil leak, that once your moving appears to be coming out the back, if you get what i mean?
if it is the engine burning oil, it would pretty much do it all the time once, not just a minute or two then stop for a bit.
hope this helps, and good luck.
ps - i woudn't bother with the service until you've eliminated a few more expensive potentials that will require your hard earned first!