This is presented as a more readable, re-formatting, of SynLube's SAE 5W-20 myth debunker page. 
Changes are minimal  and only to improve structure, grammar, and readability--all SynLube advertorial content has been omitted.

It is quite accurate in terms of the facts presented, and conclusions drawn.

C.Knight, 04/2007

SAE 5W-20 Motor Oil

The answer is simple:

You get about 1% better fuel economy, but you get 30% shorter engine life !

The above statement is based on real life experience and is comparison to SAE 5W-30 Motor Oil.

Unfortunately, in order for you to fully understand that short answer, some lengthy explanation is in order...

The SAE (Society of Automotive Engineers) developed, in June 1911, the SAE J300 standard that specifies Engine Oil Viscosity Classification.

Before SAE came up with this scheme to classify oils by their relative viscosities (in plain terms that the motoring public could easily understand) there was no simple way to tell how oil would behave in automotive engine when hot. Back then oils had no W rating, which stands for Winter. Since cars were seldom driven in winter this was not a real problem. The roads were generally impassable and vehicles usually not capable of starting when temperatures approached freezing.

The original SAE viscosity ratings were based on how quickly a specific quantity of motor oil
flowed through a test orifice when heated to operating temperature (specified as 100°C or 212°F).

The SAE Viscosity Number or Grade according to the initial SAE J300 standard was simply an average time in seconds that tested oil would take to flow through the test apparatus. Since SAE did not want to confuse the public with hundreds of numbers and the simple test yielded different times for different experimenters, it was decided to assign the grades in range steps rather than absolute test values.

Therefore the SAE Viscosity Number according to the SAE J300 standard was
 (and still is) an approximation and NOT an exact measure

• Any oil that took from 5 to 14 seconds to flow would be SAE 10;
• Oil that would take 15 to 24 seconds would be labeled as SAE 20;
• Oil that took 25 to 34 seconds would be SAE 30;
• And so on until SAE 50;

In the original SAE J300 specifications there was no SAE 5 or SAE 60 grade.

The science of Rheology was not well developed at that time, and automotive engineers were neither scientists nor physicists. Therefore it took several years before the SAE J300 staircase was translated from time measurement numbers in a crude instrument into a scientific viscosity values for viscosity expressed in Poise.

By then the J300 SAE Standard was also recognized, but not adapted by API (American Petroleum Institute) and hundreds of oil producers had thousands of cans of oil with SAE numbers already in the market place. So as not to confuse the motorists, who by then gotten used to buying motor oils identified by SAE numbers, the numbering system that by then did not relate to anything comprehensible was maintained.

As far as the author of this article could find the oldest SAE numbering system for motor oil was as follows:

The last column is not part of the SAE J300 Viscosity Standard, but rather shows the average viscosity values (and the range) of oils that were typically sold within the specific SAE Grade.

• The SAE Viscosity Numbers only indicated the oil’s ability to flow at the test temperature of 100°C;
   
• The SAE Viscosity Number did not in any way imply suitability for any purpose or quality
  or performance of the oil that carried such identification;
   
• The test was also performed ONLY on FRESH oil, so no durability or stability was ever implied;

During the early days of motoring, motor oils were pure petroleum oil produced with little to no enhancement during processing, nor did motor oils contain any additives. Therefore eventually oil marketers started to label all petroleum oils in the market place with the SAE Viscosity Numbering system numbers, so that consumers could quickly identify what viscosity the oil was when "at engine operating temperature".

This early specification was important for simple reason, oils sourced from different oil fields and different regions had vastly different viscosity index (which at that time was not yet well defined, although recognized by oil people).

Viscosity Index (VI) is nonscientific arbitrary value that simply represents the slope of inverse relationship of oil viscosity to temperature.

• All petroleum will flow slowly at room temperature, and much faster when heated up.
  therefore as the temperature is increased viscosity is decreased;
   
• This is known mathematically as inverse relationship, I.e. if one value goes up (temperature),
  then the other goes down (viscosity);

Some oils although they were thick at room temperature would flow as easily as water when hot, yet others that were not as thick at room temperature would not thin out as much. This means that two oils that appeared to have an identical viscosity at room temperature (which was usually the temperature at which the motorist would purchase or pour the oil into the engine), could have totally different viscosity when heated up.

• The early automotive engineers even then recognized the viscosity, as very important quality;
   
• And above all the viscosity when at operating temperature ("hot") was universally agreed
  to be far more important quality than viscosity at ambient temperature;
   
• This was especially important since one oil sourced from Gulf Coast, could be thick
  when cold, yet unable to protect the engine adequately when hot;
   
• By contrast another oil from Pennsylvania, a lot easier to pour when ambient,
  could be just right for automotive engine when hot;

The example of the thick when cold and really thin when hot, was oil with low viscosity index:

VI of 0 – the thick black Gulf Coast aromatic crude would behave like this.

The second example of the not so thick when cold and not as thin when hot, would be the oil with high viscosity index.

VI of 100 (then thought to be the best possible) – the amber oil which came from the oil fields of Pennsylvania and consisting of the paraffin crude that made Pennzoil and Quaker State world famous.

Although viscosity index was eventually defined by API, it was not of concern to SAE
and still today is not part of any SAE specification.

The actual viscosity at each extreme of engine operation is what automotive engineers agree on as most important specification-- it is this premise that led to the development of multigrade oils.

Over the 70 years that the SAE J300 Standard has existed, a number of shortcomings were discovered and the standard has been amended numerous times.

Although its evolution is of interest, the discussion of its exact detailed history is far beyond the scope of this article, here is in brief what has happened over the 70 years.

• SAE 60 grade was added as the need for thicker oil in aviation and heavy duty engines became apparent.
   
• SAE W grades were added in 1952 as it became apparent that engines could not be started in colder climatic conditions with some SAE 30 oils. The W (Winter) performance was originally defined as viscosity at 0°F or -11.8°C.
   
• SAE 5W and later SAE 0W grades were added as thinner economy oils needed to be defined.
   
• Additional test specifications for winter performance were added to W requirements as engines failed mechanically in cold climates immediately after initial startup, due to oil starvation.
   
• SAE 15W and SAE 25W grades were added to further narrow the performance definitions in winter climates.
   
• In 1970's minimum high temperature high shear specifications were added for performance at 150° C, when it became obvious that engines suffered from excessive wear or even seized at high speed high temperature operation such as long distance interstate driving or towing in hot summer climates.

So the changes to SAE J300 Standard were usually (until very recently) a reaction to fix an existing problem with lubricants that caused engine problems in service. This was generally due either to viscosity breakdown when hot or failure to flow when cold; in either case resulting in catastrophic engine failures.

The last few SAE J300 Standard changes were proactive. They were legislated jointly by the auto and engine manufacturers, as well as the lubricating oil producers, before problems in the field occurred, based on research tests in the laboratories--and therefore done in anticipation of problems.

Many of these specification changes were necessary because today’s cars equipped with electronic fuel injection and electronic ignition will start immediately at much lower temperatures, than vehicles made just a decade ago. Also, because of the proliferation of smaller engines with lower engine oil capacities that produce much more power that put oil under much greater mechanical as well as thermal stress.

The current SAE J300 Engine Oil Viscosity Classification Standard is tabulated below:

Revised DEC 1999

Based on our experience 99.8% of motorists have absolutely no idea what the SAE numbers on motor oil labels really mean. They assume that the simple recommendations in their vehicle owner’s manual are cast in concrete, and that the SAE viscosity of recommended motor oil can not be changed under any circumstances.

The fact that it is quite appropriate to either increase or decrease the manufacturer's recommended
motor oil viscosity, if it is appropriate for your particular operating conditions and desired engine life.

Here are some real time, as well as laboratory tested, ultimate and unchangeable truths:

1. The ideal oil viscosity for motor oil used in conventional piston engine operating at the "normal" engine operating temperature is equivalent to SAE 30. (In range of 9 cP to 12 cP @ 100°C);
    
2. If you use thinner oil (SAE 20 or less), under normal operating conditions there will be less resistance to motion due to the lower viscosity, resulting in better fuel economy. However, this gain in fuel economy does not occur without costs:
    
   1. Increase in oil consumption due to lower viscosity (can be offset by better seals);
   2. Increase in oil consumption due to higher volatility (can be offset by using synthetic oil);
   3. Decrease in engine service life due to increased boundary wear under some operating conditions
      (this will cost more per mile driven or per engine operating hour);
       
3. If you use thicker oil (SAE 40 or greater) under normal operating conditions there will be more resistance to motion due to the higher viscosity, and therefore worsened fuel economy. This loss in fuel economy is somewhat compensated for by:
    
   1. Decrease in oil consumption due to higher viscosity;
   2. Decrease in oil consumption due to lower volatility;
   3. Increase in engine service life due to reduced boundary wear and better separation of parts in relative motion;
       
4. If the ambient or operating temperature is increased from the ideal or normal (70°F/212°F) then the oil viscosity must be increased to assure same level of protection and lubricating oil film integrity;
   
   It is not just better, but a must to use SAE 40 oil at 100°F ambient and SAE 50 at 120°F ambient.
    
5. If the load is increased such as when towing, the oil viscosity must also be increased to assure the same level of protection. (use SAE 50 when towing);
    
6. If the engine speed is increased such as during long distance high speed driving in low ambient temperatures (so that the bulk oil temperature is not increased) the oil viscosity could be decreased--that is SAE 20 is preferred to SAE 30 oil (this however works only for manual transmission vehicles where vehicle speed and engine speed are proportional and higher RPM can be maintained by more frequent downshifts if necessary);
    
7. If the load is decreased then the oil viscosity can be decreased
   (when an empty tractor/semi-trailer is driven at 70 MPH on Interstate, it is OK to use SAE 30 instead of the SAE 40 that is specified and appropriate when the Tractor is hauling a maximum load at 55 MPH);
    
8. The most important factor related to long-term engine durability and component wear seems to be
   the High-Temperature / High-Shear-Rate specification shown in the last column of the SAE J300 Standard;
   
   For SAE 20 oil it is 2.6cP minimum;
   For SAE 30 oil it is 2.9cP minimum;
   For SAE 40 oil there are two specifications 2.9 cP the same as SAE 30,
       and 3.7 cP the same as both SAE 50 and SAE 60 (but why?)
   
   Well the first specification is for light-duty engines (cars that are not expected to last beyond 70,000 to 150,000 miles) and the second for heavy duty engines (that is engines which are expected to last up to 1,000,000 miles). That is why oils which are labeled as HD (Heavy-Duty) must satisfy the second SAE 40 specification of 3.7 cP.
    

[ed] The lower viscosity number for multigrade motor oils may be changed (increased generally) depending
on the lowest ambient temperatures at which you will start the engine. For most of the US and Canada 5W or 10W oils are fine,
however for warmer sections of the country 10W (or even 20W) may provide less wear at startup.

OK the final scoop on SAE 5W-20 and SAE 0W-20 oils:

For many years in the USA automotive manufacturers and importers have been subject to CAFE (Corporate Average Fuel Economy) standards that were passed by US Congress during fuel shortages of the 70's and fear of America running our of gasoline in just a few decades ([ed] which BTW, didn't happen). When enacted these laws forced US auto manufacturers to attempt to match the fuel economy of then popular Japanese Imports.

Car manufacturers get a hefty Federal fines for not meeting the CAFE MPG standards, for every 0.1 MPG by which they fail multiplied by the number of vehicles they sell. That is $5.50 per each 0.1 MPG by which the standard is missed multiplied by the number of vehicles sold in previous model year--which runs annually into millions of dollars.

Success in the car industry is measured ONLY by how many vehicles have been sold in last 10 days.

Therefore every 0.1 MPG by which you can raise fuel economy does matter, and manufacturers are quite willing to sacrifice engine durability. After all, the sooner you wear out your new car, the sooner you will buy another and that is positive impact on the 10 day sales statistics.

You will definitely get better mileage using SAE 5W-20 rather than SAE 5W-30 oil but not by much, optimistic estimates are less than 1%. The bad news is the about 30% reduction in engine life (from 100,000 miles or 10 years to 70,000 miles or 7 years) caused by the thinner oil.

Only manufacturers who have 3 years or 36,000 miles power train warranties currently recommend SAE 5W-20 oil to be used in their 2000 through 2006 model vehicles.

By contrast Mercedes-Benz that offered 4 years or 50,000 miles warranty not only specified SAE 5W-40 motor oil. And in the USA to assure that only that oil grade was used, provided periodic maintenance free to all its customers (free maintenance was offered by Mercedes-Benz from 2000 model years through 2004 model year, it was cancelled on 2005 model cars and SUV's)

All heavy-duty engine manufacturers recommend SAE 40, SAE 15W-40 or SAE 5W-40 oil.

The final choice is yours, you can get 1% better mileage or 30% longer engine life.

If you are leasing a vehicle, then the better mileage parameter is definitely more important as well as cost effective. You just do not care how long will engine last on a car that you will only operate for 24,000 to 36,000 miles. But how many gallons of fuel you will burn will make a difference.

Summary:

SAE 5W-20 motor oil is great–it yields better EPA numbers than SAE 5W-30 oil = better CAFE compliance = lower Federal Fines for not meeting minimal CAFE standards. It typically save the manufacturer about $15.00 per vehicle in CAFE fines;

SAE 5W-20 motor oil increases oil consumption–more oil gets used, which is great for oil companies everywhere;

SAE 5W-20 motor oil increases mechanical wear, reducing engine life–that way you will buy new car sooner;