What quality oil are you using ?
Are you using synthetic and did you run it in correctly? Are you a slow driver using mineral oil and never give the car a bit of stick ? We use synthetic oil and hardly ever top up between changes . Does you mechanic use the oil that gives him the best return rather than the oil that will protect your car ? A family went to a mechanic rather than have me change her oil ,she noticed the car sounded strange so I checked the oil it was not Mobil one and not the right colour ,so off to the mechanic I went and asked him to please explain .He stated sure I used the oil she gave me Mobil one see there's half a container over there left over .Now two and a half litres do not fill a Toyota seca and not when you put in a new filter .He had used the shop garage oil,junk cheap nasty .He stole from the lady in effect .Another friend traveled a long way to my home and his oil was very low when I checked its level even though he told me it was fine ,it was junk oil .So I topped up with Mobil one and when he returned the oil was still at full level when he got home .JUNK oil is expensive .Also remember after a lot of city runs the oil retains some water fuel and such , so after a good run the level will be low as the contamination is burnt off ,leaving only the oil .