Motor oil used in the engine of your Audi A4 degrades over time. The use of degraded motor oil could adversely affect the fuel economy of your vehicle. So, symptoms of motor oil degradation in your Audi A4 should not be overlooked. Replace the motor oil after a certain time interval as recommended in the owner’s manual. This slide might be helpful for you to understand the effects of motor oil degradation on the fuel economy of a vehicle.