In a car engine, oil is used to lubricate most moving parts.For your case, it seems like oil that lubricates the piston movement in the cylinder is getting into the combustion chamber and getting burnt together with the fuel. You need to do a compression test to determine whether there is compression leakage in the cylinder. This can be done in a good garage. If there is a compression leak, oil can also get past the rings if the rings or cylinders are badly worn or damaged, if the cylinders were not honed properly when the engine was built (or rebuilt), or if the rings were installed improperly.
When a newly-built engine is first started, the rings require a certain amount of time to "seat" or break-in. If the rings fail to seat properly, the engine will use oil. This may be the case if somebody applied the wrong finish to the cylinders, failed to clean and lubricate the cylinders properly before the engine was fired up, or didn't use the proper break-in procedure.
If the rings and/or cylinders are at fault, the engine will have lower than normal compression readings.
In some instances, worn rod bearings, excessive bearing clearances and/or excessive oil pressure can splash too much oil on the cylinders causing oil to get past the rings.
The cure for worn rings and cylinders is to overhaul the engine block. The cylinders have to be refinished and new rings installed to regain good oil control.
Also, make sure you are using the appropriate oil recommended by your manufacturer, check for engine oil leak under the car, and good luck!!