OIL: What Every Motorist Should Know About Motor Oil

What Every Motorist Should Know About Motor Oil

Motor oil does more than just lubricate an engine. It also forms a film on bearing surfaces that lifts and separates moving parts so they don’t touch to reduce friction and wear. The oil film also acts like a shock absorber to cushion reciprocating and rotating parts. Oil also serves as a coolant for critical engine parts such as the crankshaft bearings and valve-train. Oil also helps prevent rust and corrosion inside the engine, and helps keep surfaces clean by dissolving and carrying away dirt and varnish deposits.
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“Viscosity” refers to how easily oil pours at a specified temperature. Thinner oils have a water-like consistency and pour more easily at low temperatures than heavier, thicker oils that have a more honey-like consistency. Thin is good for easier cold weather starting and reducing friction, while thick is better for maintaining film strength and oil pressure at high temperatures and loads.

The viscosity rating of a motor oil is determined in a laboratory by a Society of Automotive Engineers (SAE) test procedure. The viscosity of the oil is measured and given a number, which some people also refer to as the “weight” (thickness) of the oil. The lower the viscosity rating or weight, the thinner the oil. The higher the viscosity rating, the thicker the oil.

Viscosity ratings for commonly used motor oils typically range from 0 up to 50. A “W” after the number stands for “Winter” grade oil, and represents the oil’s viscosity at zero degrees F.

Low viscosity motor oils that pour easily at low temperatures typically have a “5W” or “10W” rating. There are also 15W and 20W grade motor oils.

Higher viscosity motor oils that are thicker and better suited for high temperature operation typically have an SAE 30, 40 or even 50 grade rating.

These numbers, by the way, are for “single” or “straight” weight oils. Such oils are no longer used in late model automotive engines but may be required for use in some vintage and antique engines. Straight SAE 30 oil is often specified for small air-cooled engines in lawnmowers, garden tractors, portable generators and gas-powered chain saws.


Most modern motor oils are formulated from various grades of oil including base stocks refined from crude oil and recycled re-refined oil. The base stock determines the lubrication characteristics of the oil. Multi-viscosity oils contain polymer “viscosity index improvers” that alter the way the oil flows at both high and low temperatures. Multi-viscosity oils flow well at low temperature for easier starting yet retain enough thickness and film strength at high temperature to provide adequate film strength and lubrication.

A thin oil such as a straight 10W or even a 20W oil designed for cold weather use would probably not provide adequate lubrication for hot weather, high speed driving. Likewise, a thicker high temperature oil such as SAE 30 or 40 would probably become so stiff at sub-zero temperatures the engine might not crank fast enough to start.

Multi-viscosity grade oils have a wide viscosity range which is indicated by a two-number rating. Popular multi-viscosity grades today include 5W-20, 5W-30, 10W-30, 10W-40 and 20W-50. The first number with the “W” refers to the oil’s cold temperature viscosity, while the second number refers to its high temperature viscosity.

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Most vehicle manufacturers today specify 5W-20 or 5W-30 motor oil for year-round driving. Some also specify 10W-30 or 0W-20. Always refer to the vehicle owners manual for specific oil viscosity recommendations, or markings on the oil filler cap or dipstick.

As a rule, overhead cam (OHC) engines typically require thinner oils such as 5W-30 or 5W-20 to speed lubrication of the overhead cam(s) and valve-train when the engine is first started. Pushrod engines, by comparison, can use either 5W-30, 10W-30 or 10W-40.

As mileage adds up and internal engine wear increases bearing clearances, it may be wise to switch to a slightly higher viscosity rating to prolong engine life, reduce noise and oil consumption. For example, if an engine originally factory-filled with 5W-30 now has 100,000 miles on it, switching to a 10W-30 oil may provide better lubrication and protection.

For sustained high temperature, high load operation, an even heavier oil may be used in some situations. Some racing engines use 20W-50, but this would only be recommended for an engine with increased bearing clearances. Increasing the viscosity of the oil also increases drag and friction, which can sap horsepower from the crankshaft. That’s why 20W-50 racing oil would not be the best choice for everyday driving or cold weather operation for most vehicles.

The latest trend in racing is to run tighter bearing clearances and use thinner oils such as 5W-20 or even 0W-20 to reduce friction and drag.


Up to 25% of the liquid in a typical quart of oil is additive. Additives are what really make the oil and determine its performance properties. Additives extend the viscosity range of the oil, allow it to withstand high pressures and loads, handle contaminants in the crankcase, and reduce friction for improved fuel economy.

One of the most important additives is “Viscosity Index (VI) Improvers”. These help the oil maintain a consistent viscosity as temperature and load change. “Pour point depressants” are also used to prevent the oil from thickening at low temperature for easier starting.

Modern motor oils also contain detergents and dispersants to reduce varnish and sludge formation to keep the engine clean. There are also “anti-oxidants” to minimize oil burning when the oil gets hot. This also helps reduce the formation of varnish and carbon deposits inside the engine.

Rust and corrosion inhibitors are added to counteract the harmful effects of water, unburned fuel and exhaust gases that blow past the rings and enter the crankcase. This prevents the formation of acids that can pit bearing surfaces. “Foam inhibitors” are used to minimize the formation of air bubbles as the oil is churned by moving parts. “Wetting agents” help the oil stick to hot surfaces so it doesn’t run off and leave the metal unlubricated and unprotected.

Finally, there are “anti-wear” and “extreme-pressure” additives. These include zinc and phosphorus that provide wear protection when metal touches metal. Some racing oils typically have a higher dose of zinc to provide extra protection in high revving, high load applications.


The “service rating” of motor oils is classified by the American Petroleum Institute (API). The program certifies that an oil meets certain OEM quality and performance standards. The service rating is shown in the API “Service Symbol Donut” on the product label. There may also be an “API Certified for Gasoline Engines” seal on the label.











The latest service category rating for gasoline engines is SN for 2011 model year vehicles. This replaces the previous SM rating that was introduced in November 2004 for 2005 and newer engines. SN-rated oils along with the previous SM, SL and SJ ratings, are all backwards compatible and can be safely used in older engines. But the opposite is not true. Older obsolete service classifications (SH, SG, SF, etc.) may not meet OEM lubrication requirements for newer engines. Likewise, API SL oils should not be used in 2005 and later vehicles, and SJ oils should not be used in 2001 and newer vehicles.
For diesel engines, API has a separate rating system. The current category is “CI-4” (introduced in 2002 for newer diesels that have exhaust gas recirculation). The previous CH-4 (1998), CG-4 (1995), and CF-4 (1990), can all be used in older four-stroke diesel engines. CF-2 (1994) is the API classification for two-stroke diesels.

API also gives oils an “Energy Conserving” rating if the oil meets certain criteria for reducing friction and oil consumption, and improving fuel economy.

Motor oils that meet the API SN rating are equivalent to oils that meet the International Lubricant Standardization and Approval Committee (ILSAC) GF-5 specifications, which some European and Asian auto makers require. SN rated motor oils typically meet the previous GF-4 standards.

In October 2010, the ILSAC introduced a new “GF-5” motor-oil standard that provides better oil life, fewer deposits, less sludge and improved protection for turbochargers and the catalytic converter.

For 2011, General Motors announced a new oil requirement called “dexos.” GM says their new oil performance specification is better than the new GF-5 specification, which also went into effect in 2011. GM says dexos is required in all 2011 and newer GM engines, and is backwards compatible with older engines that use SM oils.

There are two versions of dexos: dexos1 for gasoline engines and dexos2 for diesel engines. The specification calls for a high quality synthetic base stock with additives that provide high temperature, high sheer characteristics to reduce friction for better fuel economy, to reduce piston ring deposits and sludge, and to extend oil life (necessary for use with GM’s Oil Life Reminder System).

Because it uses high quality synthetic base stocks, dexos and other brands of oil that meet GM’s dexos specification are more expensive than conventional motor oils. GM is licensing oil brands that meet their specifications. Pennzoil Platinum and Quaker State Ultimate Durability both claim to meet the new dexos spec in their SAE 5W-30 viscosity grade motor oils.


Regardless of an oil’s API service rating or additive package, all motor oils eventually wear out and have to be changed (actually, it’s the additives that wear out more so than the oil). As the miles add up, motor oil loses viscosity and gets dirty. The oil no longer has the same viscosity range it had when it was new, and it contains a lot of gunk (moisture and acids from combustion blowby, soot, dirt and particles of metal from normal wear). You can’t really tell much about the condition of the oil by its appearance alone because most oil turns dark brown or black after a few hundred miles of use.

The oil filter will trap most of the solid contaminants, and the Positive Crankcase Ventilation (PCV) system will siphon off most of the moisture and blowby vapors — if the engine gets hot enough and runs long enough to boil the contaminants out of the oil. Even so, after several thousand miles of driving many of the essential additives in the oil that control viscosity, oxidation, wear and corrosion are badly depleted. At this point, the oil begins to break down and provides much less lubrication and protection than when it was new. If the oil is not changed, the oil may start to gel or form engine-damaging varnish and sludge deposits — and eventually cause the engine to fail!





Oil life depends on many factors including driving conditions (speed, load, idle time, etc.), environmental factors (temperature, humidity, airborne dirt), and engine wear. As a general rule, most experts still recommend changing the oil and filter every 3,000 miles or six months, which ever comes first. Why? Because this provides the best all-round protection for the average driver.


In recent years, many vehicle manufacturers have extended their recommended oil change intervals to reduce maintenance costs for the vehicle owner — and have run into trouble. The Center for Auto Safety (www.autosafety.org) has logged over a thousand complaints about oil sludging problems from motorists who thought they were following the service intervals recommended in their owners manuals but ended up with a crankcase full of sludge.

Extended oil change intervals of 7,500 or 10,000 miles or more are based on ideal operating conditions, not the type of short trip, stop and go driving that is typical for many motorists. Consequently, most drivers should follow a “severe” service maintenance schedule rather than a “normal” service schedule to protect their engines.

Severe service includes:

* Most trips are less than 4 miles.

* Most trips are less than 10 miles when outside temperatures remain below freezing.

* Prolonged high speed driving during hot weather.

* Idling for extended periods and continued low speed operation (as when driving in stop-and-go traffic).

* Towing a trailer.

* Driving in dusty or heavily polluted areas.

Some engines, such as diesels, suffer more blowby than others and typically require more frequent oil and filter changes. For most passenger car and light truck diesels, the oil should be changed every 3,000 miles without exception — especially in turbo diesels.

Turbocharged gasoline engines also require more frequent oil changes because of the high temperatures inside the turbo that can oxidize oil. A 3,000 mile oil change interval is also recommended for all turbocharged gasoline engines.


General Motors, BMW and some of the other luxury brands have done away with recommended oil change intervals altogether and now use an “oil reminder” light to signal the driver when an oil change is needed. Some technicians now refer to this as the “Replace Engine Soon” light because of the sludging problems that have resulted from extending oil change intervals too far. The oil reminder systems estimate oil life based on engine running time, miles driven, ambient temperature, coolant temperature and other operating conditions. On some of these vehicles, the light may not come on until 10,000 miles or higher! But keep in mind that most of these engines are factory-filled with higher quality “synthetic” oil, so be sure to replace same with same when you change oil on these engines.


One of the arguments against changing oil at specific mileage or time intervals is that the oil may still be good. As long as the additive levels in the oil are adequate and the oil is not oxidizing, breaking down or contaminated with fuel or coolant, there’s no need to change it. Oil reminder lights are better than mileage/time intervals in this respect, but the light is still a guesstimate that may or may not be accurate. The only way to know for sure when the oil really needs to be changed is to test it. A sample of oil can be sent to a lab for analysis, and the report can be used to establish a change interval that reduces unnecessary maintenance. Many fleets use oil analysis to dermine oil change intervals, but for the average motorist, oil analysis is too expensive and inconvenient. The cost of the oil analysis is almost as much as an oil change.

The best approach is to use a sensor to measure the condition of the oil. An oil monitor sensor called Intellistick is available for this purpose. The sensor replaces the dipstick, and uses a bluetooth transponder to broadcast the condition of the oil to a laptop computer, PDA or bluetooth enabled cell phone.


Synthetic oils are oils that are refined to a much higher degree than ordinary oils. Synthetic oils are premium oils that generally have greater viscosity stability, lower pour points and can withstand higher operating temperatures. Synthetic oils improve cold starting, reduce friction, reduce oil consumption and improve fuel economy and performance — but they typically cost about three times as much as regular motor oil.

Some suppliers of synthetic motor oils say the higher cost of the premium quality oil can be offset by extending oil change intervals. But this would depend on the operating conditions, age and condition of the engine.

Synthetic oils are a good upgrade for most engines, but are not recommended for breaking-in newly rebuilt engines.

UPDATE Oct 30, 2007: A new green synthetic motor oil made from beef tallow (animal fat) is now available as an alternative to petroleum-based oils. The new oil is 100% biodegradeable, non-toxic, and safe (you can even drink it!). Performance is similar to other synthetic oils but it costs a little less. The product is called G-OIL.

For more information about synthetic oils, try these links:

AMSOIL Synthetic Oil
Castrol Synthetic Oil
G-OIL Green Motor Oil (biodegradeable synthetic oil from beef tallow)
Mobil1 Synthetic Oil
Pennzoil Synthetic Oil
Quaker State Synthetic Oil
Redline Synthetic Oil
Royal Purple Synthetic Oil
Valvoline Synthetic Oil

For motorists who want the benefits of a synthetic oil in a less expensive product, there are “synthetic blends” that mix 20 to 25% synthetic oil with conventional oil. Blends cost about a dollar a quart more than ordinary oil, and provide many (but not all) of the benefits of a full synthetic.

There are also oils that have special additive packages for specific applications such as large, heavy Sport Utility Vehicles (SUVs), turbocharged engines (extra anti-oxidants) and high mileage engines (extra viscosity improvers and anti-wear additives).









* Drain the oil while it is hot. Contaminants will be in suspension and drain more easily from the engine.

* Always replace the filter when changing the oil

* Wipe some oil on the filter gasket so the seal won’t stick or tear.

* Hand tighten the filter about 1/2 to 3/4 of a turn after the gasket makes contact. Over-tightening may damage the threads or gasket, and make the filter difficult to remove the next time the oil is changed. Under-tightening may allow the filter to work loose and leak.

* If the oil is badly contaminated or sludged, the crankcase should be cleaned and flushed before the engine is refilled with oil.

* Always check the oil level after refilling the crankcase. Start the engine, then shut it off and check the oil level after several minutes. It should be at the full mark on the dipstick. Most engine hold about four quarts of oil, plus half a pint to almost a quart for the filter (depending on the size of the filter). Overfilling can cause oil foaming and leaks. Under-filling may cause a loss of oil pressure and engine damage!

* Dispose of your old used oil properly. Save it in a container and take it to an auto parts store or other facility that recycles oil. Do NOT dump it on the ground, down a storm sewer or anyplace else where it can contaminate ground water.

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