Motor oil FAQs

Motor oil breaks down over time. When it breaks down, it loses its effectiveness and can no longer properly protect your engine.

In addition to lubricating an engine's moving parts, motor oil is designed to carry combustion byproducts away from the pistons and cylinders. It is designed to deal with the small amounts of water that form as the engine heats and cools, and to collect the dirt and dust that enter the engine through the air-intake system. It also handles acids that are formed by the reaction between water and other contaminants. Sometimes there are even fuel leaks (fuel dilution) or coolant leaks that get into the oil system.

As a car is driven, the level of contamination in the motor oil constantly increases. The oil filter removes particles as the oil passes through the filter, but over time an oil's additives are used up and the oil itself can start to degrade (oxidize or thicken). At that point, the oil can no longer do its job and must be changed.

The rate at which contamination and additive depletion occurs depends on many variables. One of these is driving conditions, which vary greatly and have a direct effect on the useful life of the oil. Other factors include the precision of ignition, fuel injection or carburetion adjustments, air cleaner service, and the general mechanical condition of the engine.

Oil should be changed before the contamination level reaches the point where engine damage can result. Because it is difficult for the individual motorist to determine when the contamination level is too high, automobile manufacturers provide recommended oil change intervals. These change recommendations vary by model year and manufacturer. Recommended intervals and mileage limits also vary with the type of service under which a car operates. More frequent oil changes are recommended for severe service.

Not necessarily. "Straight weight" refers to an oil's viscosity and usually indicates an absence of viscosity index improvers. "Non-detergent" refers to the absence of detergent additives that are used to handle combustion byproducts.

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, or leaking.

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.

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.

If the cloud is relatively blue or blue/black, it may indicate that oil is being burned along with the fuel. The possible cause may be either worn piston rings or an oil viscosity that is too low.

If the cloud is black, it indicates that excess fuel is being burned.

If the cloud is white, it may simply be the moisture in the cold engine and exhaust system being burned off. If there's a lot of white smoke and it continues for a long time, you may have an internal coolant leak.

Engine oils are currently classified by a two-letter code. Gasoline engine oil categories start with the letter S (originally designated "Spark Ignition" engine oils, we now associate the S with "Service"). Diesel engine oil categories start with the letter C (originally designated "Compression Ignition" engine oils, we now associate the C with "Commercial").

The second letter is simply a sequential designation of improving quality levels over time. In other words, when a new industry quality level is established, the next letter of the alphabet is used (so SN replaced SM). The letters "I" and "K" were purposefully skipped to eliminate potential confusion with other commonly used designations.

The viscosity index (VI) number is a measure of the relative change in viscosity of oil over a temperature range. 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 it 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.

Your oil warning light can come on for a number of reasons, including low oil level, a failing oil pump, a faulty oil-pressure sensor, blockage in the oil system, excessive foaming of the oil and more. In all cases, you should shut down your engine as quickly as it is safe to do so. Continuing to operate your engine with low oil pressure can result in serious engine damage.

Water can get into the oil two ways. You generally don't have to worry about one of the ways if you drive your car enough to burn off the water during normal driving. This drives off the moisture that simply comes from your engine breathing when it is not being used (cold air enters a hot engine and water condenses) and from combustion byproducts. The second more destructive route is through a coolant leak due to a bad gasket, an engine crack, etc. This amount of moisture is generally going to cause serious engine issues including loss of power, oil sludging, etc. Consult your mechanic when in doubt.

Mobil Delvac 1™ ESP 5W-40 is a fully synthetic exceptional performance heavy duty diesel engine oil that helps extend engine life while providing long drain capability and potential fuel economy for modern diesel engines operating in severe applications. Mobil Delvac 1™ ESP utilizes state-of-the-art technology to deliver exceptional performance in modern low emission engines, including those with Exhaust Gas Recirculation (EGR) and Aftertreatment systems with Diesel Particulate Filters (DPFs) and Diesel Oxidation Catalysts (DOCs). Therefore, we believe it will be a good oil for the high mileage vehicles when combined with the proper care for your vehicles as recommended by your manufacturers' guidelines. You can also use Mobil 1™ Turbo Diesel Truck 5W-40 as an alternate.

In simple language, the first part of the viscosity designation (W grade) is an indication of the product's ability to help an engine crank and start and for the engine to pump the lubricant. The lower the number (0W is the lowest), the lower the temperature at which the product can be used. So the W grade is related to the lowest temperature your engine sees when you start the engine on the coldest morning of the year. But also keep in mind that a lower W grade pumps and helps an engine to start better than a higher W grade. The second part of the viscosity grade is related to the viscosity your engine sees at operating temperature. In this case, a higher number is a higher viscosity grade and provides more viscous oil at operating temperature than lower viscosity grade oil. It is not safe to assume that a higher viscosity oil is always better for your engine because other factors, such as engine design, fuel economy and power, are also related to operating viscosity. You should always consult your owner's manual for the right oil to use for your particular engine. For more information on the viscosity grade specifications, a quick search of the Internet will identify several websites where information about SAE J300 (Viscosity Properties Test) can be found.

Motor oil is for your vehicle's engine. A couple of differences are the following: Motor oil is designed to deal with the products of combustion, whereas automatic transmission fluid (ATF) does not see contaminants from fuel burning. An ATF is basically a closed system and the lubricant must last for a long period of time. Motor oil must be drained to remove contaminants after a relatively short time and/or mileage compared to an ATF. Similar additive component types are used, but at different levels and different chemistries. In an ATF, clutch friction is a very important consideration. Transmission fluid is also used by your steering system to keep its parts moving smoothly.

Viscosity is a measure of a fluid's resistance to flow. A fluid with low viscosity flows easily and is Motorcycle oils and passenger car oils are very similar, with the exception of a couple of areas that are key to motorcycle operation. The first area concerns common sumps, or the use of motor oil, to lubricate and cool the transmission. As you know, in a passenger car the transmission is lubricated by an ATF fluid, which has frictional properties required for transmission operation. In a motorcycle, where the motor oil may lubricate the transmission, a motor oil that does not have the same level of friction modification (for fuel economy) of a typical passenger car motor oil will provide better transmission performance in terms of transmission lock-up and slippage. So motorcycle motor oil does not contain the friction modifiers of a passenger car motor oil. The second area of concern for motorcycle motor oils is that they tend to shear (break down viscosity) more quickly than a typical passenger car. Mobil 1™ motorcycle oils are designed to provide exceptional protection against viscosity loss.

ExxonMobil recommends a five-year maximum shelf life for engine oils, including Mobil 1™ synthetic motor oil.

The instructions are very easy to follow: Simply change the oil as you would normally. Any “confusion” is just myth. You can switch from conventional motor oil to Mobil 1™ synthetic oil (and back again, if you want) without following any special procedures. There are two exceptions to this: a higher-mileage engine that has never used synthetic motor oil, or an engine that has used conventional motor oil and been poorly maintained. In these cases, you should still follow the same basic oil-change procedures (drain the old oil, remove the old oil filter, put in new Mobil 1 motor oil and put on a new oil filter), but you should follow a regimen of one or two shortened oil-change intervals. For instance, let’s say that your regular oil change interval is 5,000 miles. If you’re switching to Mobil 1 under either of the circumstances mentioned above, make your next Mobil 1 oil change in 2,500 miles, your third Mobil 1 oil change 3,500 miles after that, and then follow your normal 5,000 mile oil-change interval.

The reasoning behind this staggered interval is that a high-mileage engine, or one that has seen infrequent oil changes, will likely have a considerable buildup of sludge and deposits. Mobil 1 motor oil will help clean the engine as you drive, but it will have to work much harder in a very “dirty” engine, and so it is best to change the oil more frequently for those first few thousand miles. After that, you can rest assured that Mobil 1 motor oil is continuing to keep your engine running clean and well lubricated for mile after mile.

In general, oils should be compatible with each other. It is not likely that you would form gel by mixing the two oils. However, we would not recommend mixing oils as a general practice because oils are complex mixtures of additives and base oils that can be destabilized. Also, why reduce the outstanding performance of Mobil 1™ synthetic motor oils by adding “regular oil”? Is it economics? You would be better to run all Mobil 1 synthetic motor and run it longer than mixing it with “regular oil.”

We recommend checking the oil level either before turning on the engine or 5 to 10 minutes after shutting down so you can have all the oil in the oil pan to get an accurate measurement.

Synthetic oils typically provide better protection than conventional oils, but switching back and forth between full synthetic and conventional oil will not damage the engine. Of course, this depends on the current engine condition and the quality of the conventional oil being used.

Ester oil is synthetic base oil that has been chemically synthesized. Typically ester oils are used in passenger car air-conditioning compressors, refrigerators and other industrial applications. Esters are one of the classes of synthetics that have been used in Mobil 1™ oils  as well. Esters are stable molecules, provide good solvency, and provide very good low-temperature and high-temperature performance in engine oils.

There are certain oils in the market today that use EP (extreme pressure) additives in their motor oil that are really designed for gear oils and not motor oils. Extreme pressure additives are typically not used in motor oils for a number of reasons, but the most important reason is that they can cause engine corrosion over time. The rigs being used in these demonstrations are primarily designed for industrial applications like gear oils where extreme pressure is a necessary and important performance feature. These demonstration rigs have very little to do with modern engines, which is why market leading oils in the industry perform poorly in these tests.

The formulation and quality level of our motor oils are the same wherever the product name is the same, regardless of the package size in which it is sold.