May be piston rings are weak. Somewhat reduction in oil level after driving several thousand kilometres is also normal. Do you see blue smoke coming out when you start the car after a long standing? Also get under the car and check for any oil leakage.
If you feel that engine power is fine. You can choose not to overhaul its engine now and keep it going. Just keep the oil can in your car and pour it carefully if the level start reaching lower dipstick mark. Never pour it exceeding the designed capacity as you will damage the engine. Running engine with a little lesser oil does not harm it but excess does.