If your vehicle is showing having to top off engine oil frequently without seeing any drips on the driveway, here are the most likely causes and the parts you'll need to fix each one.
A stuck or clogged Positive Crankcase Ventilation (PCV) valve can cause the engine to suck oil vapor directly into the intake manifold, where it is burned during combustion. This is a very common and inexpensive issue to fix.
Using an engine oil that is too thin for your specific vehicle or climate can lead to excessive evaporation and burning inside the cylinders. This often happens if the wrong oil was used during a recent oil change.
The seals around the engine valves harden and degrade over time, allowing oil to drip down into the combustion chamber, especially after the car has been sitting. This typically produces a noticeable puff of blue smoke when you first start the engine.
Worn or carbon-caked piston rings fail to scrape oil off the cylinder walls, allowing it to bypass the rings and burn during combustion. This is common on high-mileage engines and often causes continuous blue smoke under heavy acceleration.
If your vehicle is turbocharged, worn internal seals can leak oil directly into the intake or exhaust system where it burns off rapidly. This can consume a massive amount of oil very quickly without dripping on the ground.