It's time to change the oil in your car - what grade of oil, or viscosity, should you use?

Viscosity is a measure of a fluid's resistance to flow. A fluid with low viscosity flows easily and is often called "thin." Water is an example of a fluid with a relatively low viscosity. A fluid with high viscosity is often described as "thick." Maple syrup is an example of a fluid with a relatively high viscosity. Typically, your owner's manual will advise you of the best decision to make when choosing an oil viscosity for your vehicle, though if you live in challenging climates, you will need to choose an oil viscosity better suited to extremely warm or cold temperatures.

What does a viscosity index number mean?
The viscosity index (VI) number is a measure of the relative change in viscosity of oil over a temperature range. According to Mobil Oil, the HIGHER the viscosity index, the SMALLER the viscosity change over temperature. The VI is not related to the actual viscosity or SAE viscosity, but is a measure of the rate of viscosity change.

The VI number is typically used only as an indicator. The actual performance results of low-temperature pumpability tests and high-temperature wear tests of a motor oil are better predictors of good performance in an engine.

Generally, multigrade oils (0W-40, 10W-30, etc.) will have high viscosity indexes. Monograde oils (SAE 30, 40, etc.) will have lower viscosity indexes.

What role does viscosity play in a motor oil?
Engineers work to establish an optimal viscosity for an oil, based on load and speed conditions. They balance lighter - or low-viscosity - oil, which provides little resistance to motion thereby saving fuel and efficiently transferring horsepower, with a heavier - or high-viscosity - oil that resists being squeezed out of the contact area between metal surfaces.

The complicating factor is that the viscosity of an oil varies with changes in temperature - thinner when hot, thicker when cold. At low temperatures, we need the motor oil to flow readily (not thicken too much or gel). At high temperatures, we need the motor oil to keep from becoming too thin and allowing metal-to-metal contact. Therefore, engineers developed multigrade motor oils.

Click here to watch a Mobil 1 Oil video demonstrating how choosing the correct viscosity synthetic oil can protect your vehicle even in extreme cold.

What makes synthetic oil superior to conventional oil?
All motor oils are made up of base oils and additives. In general, fully synthetic motor oils contain non-conventional, high-performance fluids and a system of additives to help prevent wear, keep an engine clean, flow easily, maintain viscosity, prevent rust, and reduce friction. The result is a lubricant that remains liquid and slippery under the widest possible range of temperatures and engine conditions.

Historically, conventional oils lack the performance of synthetic oils in the areas of low-temperature performance and high-temperature oxidation stability. Conventional motor oil is made up of an inconsistent mixture of long and short chains of carbon and hydrogen atoms. In the extreme heat of an engine, short-chain molecules can evaporate, and unstable molecules can oxidize and break down. Conventional oils also contain much greater amounts of impurities, such as sulfur, reactive and unstable hydrocarbons, and other undesirable contaminants that cannot be completely removed by conventional refining of crude oil.

Mobil 1 uses high-performance fluids, including polyalphaolefins (PAOs), along with a proprietary system of additives. Each Mobil 1 and Mobil 1 Extended Performance viscosity grade use a unique combination of synthetic fluids and selected additives in order to tailor the viscosity grade to its specific application.