Oil consumption on Porsche 911 engines (not too dissimilar to your average lycoming) is acceptable at a level greater than 1Litre per 100 miles..! I found this almost unbelievable, but aparently true.
If my aero engine were using any more than 1 litre in 4 hours, I would start to worry. It is currently around 1 litre/10 hours.
GG