Does Engine Oil Get Thinner At Higher Temperatures?

When changing the oil in a gasoline or diesel vehicle, it is of utmost importance to adhere to the recommended oil change intervals and use the correct grade and classification of oil. Diesel engines generally use thicker oil to handle high combustion temperatures, while most gas-fueled vehicles require multi-grade oils. These formulations differ in viscosity — how easily they flow — at different temperatures. They are thicker when cold and thin out as they warm up. If the oil remains too thick at operating temperature, it won't spread properly to all the engine's internal parts, leading to excessive friction and wear.

Plenty of damage can be done in those precious seconds that go by between starting the engine and getting it up to running temperature. Oil that flows readily when cold will better protect an engine during these critical first few seconds and when it's very cold outside. But, like any other fluid, oil gets thinner when exposed to heat. Too much heat can cause oil molecules to break down and lose their viscosity. In extreme cases, oil can become too thin to protect the engine from metal-on-metal friction.

Modern synthetic or synthetic-blend engine oil contains advanced additives and viscosity-index (VI) improvers or viscosity modifiers. These additives contain polymer-based molecules that react to changes in temperature and enable the oil to contract at low temperatures without disrupting the viscosity. This provides robust protection from wear during cold starts.

VI improvers can also expand the oil's molecular chain at higher temperatures, making the oil thinner and less viscous. However, intense heat can break down the VI modifier molecules into smaller ones, which will increase the oil's viscosity. Synthetic oils are more resistant to this breakdown than conventional oil, and will be effective for longer periods and at higher temperatures. This further highlights the importance of not skipping oil changes and using the highest-quality oil that your budget allows.

Engine oil can perform other duties inside the engine besides lubricating moving parts. Some modern formulations contain detergent additives to help clean carbon deposits from the inside of the engine and absorb the tiny metal particles that result from normal engine wear. This action can also help lower combustion temperatures to improve efficiency, especially in demanding scenarios like towing, racing, or climbing steep grades. However, detergent additives and VI improvers will begin to break down as the oil ages, and oil also gets thicker over time due to the accumulation of sludge. It's essential to check your oil each time you refuel your car and change the oil and filter according to the manufacturer's recommendations. If you can't find that information in your owner's manual, call your dealership's service department and get it directly from them.