MOTOR OIL VISCOSITY RATINGS
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.

MULTI-VISCOSITY MOTOR OILS
Most modern motor oils are formulated from various grades of oil so the oil will have the best characteristics of both thick and thin viscosity oils. 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.

Note: Motor oils that have a wider range viscosity rating such a 5W-30 are blended with more base stocks and additives. Because of this, it may be harder for a wider range oil to remain in grade as the miles accumulate (which is why GM does NOT recommend using 10W-40 motor oil -- they say it breaks down too quickly and does not say in grade as long as 10W-30 or 5W-30). Also, an oil with a lower winter rating like 5W-30 or 0W-30 will contain a higher percentage of thinner base stock oil. This requires more viscosity improver additive to achieve a the same high temperature rating of a 10W-30 or straight 30 oil.

Most vehicle manufacturers today specify 5W-30 or 10W-30 motor oil for year-round driving. Some also specify 5W-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 90,000 miles on it, switching to a 10W-30 oil may provide better lubrication and protection. The thicker oil will maintain the strength of the oil film in the bearings better so the engine will have more oil pressure. This will also reduce engine noise and reduced bearing fatigue (which can lead to bearing failure in high mileage engines).

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-30 or 5W-20 to reduce friction and drag.

API Motor Oil Service Classifications
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 "SM", introduced in November 2004 for 2005 and newer engines. SM-rated oils along with the previous "SL" (2001) and "SJ" (1997) ratings, are 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 current API SM rating may also meet the new International Lubricant Standardization and Approval Committee (ILSAC) "GF-4" specifications, which some European and Asian auto makers require.

How Often Should You Change Oil?
By Larry Carley copyright 2006 AA1Car.com

How often should you change the oil in your engine? On most new vehicles, the factory recommended interval for changing the oil and filter is once a year or every 7,500 miles in passenger car and light truck gasoline engines. For diesel engines and turbocharged gasoline engines, the recommended interval is typically every 3,000 miles or six months.
If you read the fine print in your owners manual, however, you'll discover that the once a year, 7,500 mile oil change is for vehicles that are driven under "ideal" operating circumstances. What most of us think of as "normal" driving is actually "severe service" driving.
Severe service driving includes:
Frequent short trips (less than 10 miles, especially during cold weather)
Stop-and-go city traffic driving
Driving in dusty conditions (gravel roads, etc.)
Driving at sustained highway speeds during hot weather.
For severe service driving (which is what most of us do), the most common recommendation is to
change the oil every 3,000 miles or six months (which ever comes first).
For maximum protection, many people change their oil every 3,000 miles or three to six months regardless of what type of driving you do.
This probably isn't necessary on a newer vehicle with a low mileage engine that is used for highway driving or long distance commuting. But it won't anything either. A new engine with little or no wear can probably get by on 7,500 mile oil changes. But as an engine accumulates miles, blowby increases. This dumps more unburned fuel into the crankcase which dilutes the oil. This causes the oil to break down. So if the oil is not changed often enough, you can end up with accelerated wear, oil sludging and all the engine problems that come with it: loss of performance & fuel economy, and increased emissions and oil consumption.
To find out what ASE certified Master Technicians said about oil change intervals in a June 2006 survey, Click Here.

OIL REMINDER LIGHTS
General Motors and a number of European vehicles 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. On some of these (Volkswagen, Audi, Mercedes Benz, BMW and Volvo), a scan tool or a special service tool is required to reset the oil service reminder light. On others, there is a manual procedure for resetting the oil reminder light.
DO NOT IGNORE THE LIGHT! When the light is on, it is time to change the oil.
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 the oil on these engines is changed.

WHY OIL NEEDS TO BE CHANGED
Regardless of what type of oil you use (conventional, synthetic or synthetic blend), 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. Eventually this will cause the engine to fail!

MOTOR OIL ADDITIVES
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.

AFTERMARKET OIL ADDITIVES & CRANKCASE TREATMENTS
Engine oil additives and treatments include friction reducers/modifiers, products that help reduce oil burning, engine flushes to remove carbon. varnish and sludge deposits inside the crankcase, and moly and teflon-based crankcase additives that help reduce wear and extend engine life. These products can't work miracles, but they can reduce oil consumption in older engines and help protect newer engines against premature wear.

Most vehicle manufacturers do NOT recommend using any type of aftermarket oil additive. Quality motor oils should contain all the additives that are necessary for normal service and to maintain oil lubricity between normal oil change intervals. On the other hand, most vehicle manufacturers do NOT prohibit the use of aftermarket oil additives in newer engines.

Aftermarket oil additives make the most sense for older, high mileage engines that are burning or leaking oil, or that may be used for severe service or racing.

In recent years, the level of phosphorus has been reduced in motor oil because phosphorus can contaminant the catalytic converter if the engine is burning oil. Phosphorus is an anti-wear additive, so there is some concern that lower levels of this essential additive may increase wear in older engines that have flat tappet cams instead of roller cams. To help protect older engines with flat tappet cams against wear, using an aftermarket oil additive that contains phosphorus or other anti-wear additives may help prolong engine life.