If you haven't got any leaks then it must be burning it, quite common on these engines to burn a bit of oil when I had a issue many years ago Vauxhall would only look at my car if it used more than 1litre with 1000 miles.
Valve guides can get worn and stem seals as well as the rings getting gunked up, might be worth getting someone to view the exhaust while you blip the revs. See if there is any visible oil burning.
Also what oil are you using?