Yes, engine oil can and does evaporate over time, but it's a very slow process. Modern engine oils are specifically formulated to have low volatility, meaning they resist turning into vapor at normal operating temperatures. The primary cause of oil loss in a car is typically burning oil in the combustion chamber or leaks, not simple evaporation from the crankcase.
The rate at which oil evaporates depends on its volatility, which is measured by the Noack Volatility Test. This test determines the percentage of oil that evaporates when heated to 150°C (302°F) over one hour. A lower percentage is better, indicating a more stable oil that's less prone to vaporization. High-quality synthetic oils generally have superior volatility ratings compared to conventional oils.
Here’s a comparison of typical Noack Volatility values for different oil types:
| Oil Type | Typical Noack Volatility Range | Key Characteristics |
|---|
| Premium Synthetic Oil | 5% - 10% | Highest resistance to evaporation; excellent for high-performance or high-temperature engines. |
| Standard Synthetic Blend | 10% - 13% | Good balance of volatility control and cost. |
| Conventional Mineral Oil | 15% - 20% | Higher evaporation rate; more frequent top-offs may be needed. |
While evaporation is a minor factor under normal conditions, it can become significant in extreme situations. If you frequently drive under high-stress conditions—such as sustained high speeds, towing heavy loads, or in very hot climates—the engine runs hotter, accelerating oil vaporization. This is why you might notice a slight decrease in oil level on the dipstick between changes without any visible leaks.
If you're consistently losing a quart of oil every 1,000 miles or less, evaporation is unlikely to be the culprit. You should investigate more common issues like worn piston rings, valve stem seals, or external gasket leaks. For most drivers, using a high-quality, low-volatility oil and adhering to the manufacturer's recommended change intervals will minimize any noticeable loss from evaporation.
