An engine that needs oil topped up between scheduled changes, or exhaust smoke with a visible blue tint (especially on startup or under hard acceleration), both point to oil being burned inside the combustion chamber rather than staying where it belongs.
Oil reaches the combustion chamber one of two ways: past worn piston rings (from below) or past worn valve stem seals (from above). Either path lets oil burn alongside the fuel-air mixture, which both consumes the oil over time and produces that telltale blue-tinted exhaust smoke. A small amount of consumption is normal on any engine as it ages; the issues below are specifically the documented cases where consumption happens much earlier, or much faster, than normal wear would predict.
Several of the cases below were significant enough that manufacturers extended warranty coverage or faced consumer complaints at scale — this isn't simply 'old engines burn a little oil,' but specific, identifiable design-era issues on specific engines.
This symptom shows up across several unrelated engines and drivetrains — each has its own root cause and its own full writeup.
Check oil level yourself with the dipstick rather than relying solely on a dashboard warning light, since consumption can outpace what some sensors are calibrated to flag.
Ask for documented oil consumption history on any used car you're evaluating, and check whether any manufacturer extended warranty or goodwill repair applies to your specific engine and production year.
Watch for blue-tinted exhaust smoke specifically on a cold start and under hard acceleration during any test drive — it's the clearest visual confirmation.