Motor Oil 101

• INTRODUCTION
• VISCOSITY
• WEIGHTS
• MULTI-VISCOSITY OILS
• VISCOSITY INDEX
• FLASH POINT
• POUR POINT
• %SULFATED ASH
• % ZINC
• TABLE OF PROPERTIES
  • 20W-50
  • 20W-40
  • 15W-50
  • 5W-50
  • 5W-40
  • 15W-40
  • 10W30
  • 5W-30
• VISCOSITY IMPROVERS
• DINO_OIL
• SYNTHETIC OIL
• ADVANTAGES OF SYNTHETIC OIL
• DISADVANTAGES OF SYNTHETIC OIL
• DINO/SYNTHETIC BLENDS
• WHAT I USE

INTRODUCTION    
Choosing the best motor oil is a topic that comes up frequently in discussions between motorheads. The following article is intended to help you make a choice based on more than the advertising hype.

Oil companies provide data on their oils most often referred to as “typical inspection data.” This is an average of the actual physical and a few common chemical properties of their oils. This information is available to the public through their distributors or by writing or calling the company directly. I have compiled a list of the most popular premium oils so that a ready comparison can be made. If your favorite oil is not on the list, get the data from the distributor and use what I have as a database.

This article looks at six of the most important properties of a motor oil readily available to the public: viscosity, viscosity index (VI), flash point, pour point, % sulfated ash, and % zinc.

VISCOSITY    
Viscosity is the measure of how thick an oil is. This is the most important property for an engine. An oil with too low a viscosity can shear and loose film strength at high temperatures. An oil with too high a viscosity may not pump to the proper parts at low temperatures and the film may tear at high rpm.

WEIGHTS    
The weights given on oils are arbitrary numbers assigned by the S.A.E. (Society of Automotive Engineers). These numbers correspond to “real” viscosity, as measured by several accepted techniques. These measurements are taken at specific temperatures. Oils that fall into a certain range are designated 5, 10, 20, 30, 40, 50 by the S.A.E. The W means the oil meets specifications for viscosity at 0 F and is therefore suitable for Winter use.

MULTI-VISCOSITY OILS    
Multi viscosity oils have polymers added to a light base (5W, 10W, 20W), which prevent the oil from thinning as much as it warms up. At cold temperatures the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up, the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. The result is that at 100 degrees C, the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot.

Multi-viscosity oils are one of the great improvements in oils, but they should be chosen wisely. Always use a multi-grade with the narrowest span of viscosity that is appropriate for the temperatures you are going to encounter. In the winter, base your decision on the lowest temperature you will encounter; in the summer, the highest temperature you expect.

10W-40 and 5W-30 require a lot of polymers (synthetics excluded) to achieve that range. The polymers can shear and burn, forming deposits that can cause ring sticking and other problems. This has caused problems in diesel engines, but fewer polymers are better for all engines. The wide viscosity range oils, in general, are more prone to viscosity and thermal breakdown due to the high polymer content. It is the oil that lubricates, not the additives. Oils that can do their job with the fewest additives are the best.

Very few manufactures recommend 10W-40 any more, and some threaten to void warranties if it is used. It was not included in this article for that reason. 20W-50 is the same 30 point spread, but because it starts with a heavier base it requires less viscosity index improvers (polymers) to do the job. AMSOIL can formulate their 10W-30 and 15W-40 with no viscosity index improvers but uses some in the 10W-40 and 5W-30. Mobil 1 uses no viscosity improvers in their 5W-30 and, I assume, the new 10W-30. Follow your manufacturer’s recommendations as to which weights are appropriate for your vehicle.

VISCOSITY INDEX    
Viscosity Index is an empirical number indicating the rate of change in viscosity of an oil within a given temperature range. Higher numbers indicate a low change, lower numbers indicate a relatively large change. The higher the number the better. This is one major property of an oil that keeps your bearings happy. These numbers can only be compared within a viscosity range. It is not an indication of how well the oil resists thermal breakdown.

FLASH POINT    
Flash point is the temperature at which an oil gives off vapors that can be ignited with a flame held over the oil. The lower the flash point the greater tendency for the oil to suffer vaporization loss at high temperatures and to burn off on hot cylinder walls and pistons. The flash point can be an indicator of the quality of the base stock used. The higher the flash point the better. 400 F is the minimum to prevent possible high consumption.

POUR POINT    
Pour point is 5 degrees F above the point at which a chilled oil shows no movement at the surface for 5 seconds when inclined. This measurement is especially important for oils used in the winter. A borderline pumping temperature is given by some manufacturers. This is the temperature at which the oil will pump and maintain adequate oil pressure. This was not given by a lot of the manufacturers, but seems to be about 20 degrees F above the pour point. The lower the pour point the better.

%SULFATED ASH    
Percent sulfated ash is how much solid material is left when the oil burns. A high ash content will tend to form more sludge and deposits in the engine. Low ash content also seems to promote long valve life. Look for oils with a low ash content.

% ZINC    
Percent zinc is the amount of zinc used as an extreme pressure, anti-wear additive. The zinc is only used when there is actual metal to metal contact in the engine. Hopefully the oil will do its job and this will rarely occur, but if it does, the zinc compounds react with the metal to prevent scuffing and wear. A level of 0.11% is enough to protect an automobile engine for the extended oil drain interval under normal use. Those of you with high revving, air cooled motorcycles or turbocharged cars or bikes might want to look at the oils with the higher zinc content. More doesn’t give you better protection; it gives you longer protection if the rate of metal to metal contact is abnormally high. High zinc content can lead to deposit formation and plug fouling.

VISCOSITY IMPROVERS    
Why buy 10W-30 when there is 5W-30 or 10W-40 available? Is there something “wrong” with oils with a larger range? Nothing’s really “wrong” with larger ranges, but the fact that an oil has a larger viscosity range means that it uses more viscosity improvers.

Viscosity improvers (VIs for short) are fairly large molecules which, at low temperatures, are “curled” into little balls and don’t thicken the oil. At higher temperatures, the VIs “uncurl” into long chain molecules which give the oil greater viscosity. Thus, a 5W-30 behaves like a 5W oil at low temperature, and thickens at high temperature into a 30W viscosity.

The drawback of VIs is that because they are long and complex molecules, they are very susceptible to shear as oil circulates within the engine. VI’s will suffer breakdown and lose their ability to perform their task. The more VI’s an oil uses, the more the oil is subject to this breakdown; losing its ability to provide the necessary viscosity improvement.

DINO OIL    
Dino oil begins with a base material which is separated from other various crude oil cuts by its boiling range. Various components in crude oil boil off at different temperatures, and material from various ranges goes to a variety of end products such as: kerosene, gasoline, diesel, jet fuel, lube oils, asphalt, etc.

The point being that dino oil base is not a particular chemical species, but a myriad of species, with the only thing in common a similar boiling range. Once this crude cut is split fine enough to be a particular type of lube oil – say automotive engine oil targeted for a particular viscosity range – various additive packages are added. Some of these additive packages are viscosity improvers, corrosion inhibitors and additives to improve filming.

A common process in the refining of crude oil is called “cracking”. In this process, big molecules are heated and “cracked” into smaller molecules. The smaller molecules vaporize and are condensed and collected for further processing. When this happens, the various bits left behind can react with each other and form cross-linked molecules, tars, that are resistant to cracking, but are also not good at lubrication. This chemical reaction takes place to oil in your engine. Light components are generated which boil off, and tars are generated and left behind. Eventually the reaction can continue to the point of making varnishes; not like you put on your sailboat, but really heavy junk that solidifies in cooler areas of the engine on various engine parts.

Changing your oil not only removes the acids and other combustion by-products that have collected in the oil, but it also allows for removal of broken down oil.

SYNTHETIC OIL    
It sounds more exotic than it is. It is not oil made from some space-age fabric or some such. It is an oil which has purpose-made molecules as the oil base rather than a range of molecules which share a similar boiling point. These molecules are designed and manufactured specifically as an oil base. The development engineer can choose how to engineer the tradeoffs between lubricity, thermal reactivity and other parameters. Similar additive packages are added, but typically fewer additives are needed since much of the final properties can be designed into the base molecule.

Synthetic oil is more expensive because it has to be manufactured rather than just separated from a crude cut.

It can be run for longer periods of time between oil changes because it has better thermal stability. The Mobil 1 commercials where they put dino oil and synthetic oil in pans and cook them until the dino oil breaks down are not hype.

So should you run longer intervals with synthetic oil? Well, there are still going to be acids and other combustion products in the sump over time. These are not removed by a filter. And even the stuff that is removed by the filter will eventually load the filter to the point that its efficiency drops. So I think that synthetics should still be changed at reasonable intervals.

ADVANTAGES OF SYNTHETIC OIL    
Better lubrication properties. Less breakdown over time. These two mean much better relative lubrication properties of the base oil in the latter parts of a particular oil change interval. But, does the improvement really reduce wear significantly? Properly changed dino oil with good filters will enable your engine to operate for a long time without concern for poor-lubrication-induced failures, so is synthetic adding any value? Finally, there is the benefit of lower friction or efficiency. But I’m not sure how well this can be quantified. Would it really cut your 1/4 ET? Not so you could notice.

DISADVANTAGES OF SYNTHETIC OIL    
First is cost. Are you really going to run a long time with acids and combustion products collecting in our oil? Folks that use oil analysis to determine oil change frequencies can evaluate this tradeoff , but I can’t.

The second disadvantage is that synthetic oil usually is lower in viscosity at low temperatures. It will have a tendency to drain down faster. On the other hand, it usually has better filming properties, so this probably is not a concern. You may hear your engine more on cold startup, but it may not be receiving any more harm. Third, it also tends to be a bit thinner at high temperature. This can exacerbate oil leaks and oil burning. Since the oil is more expensive, it makes the leaks/burning more expensive.

DINO/SYNTHETIC BLENDS    
When you get into blends you are getting into variables that are controlled by the particular recipe that each manufacturer is using.

I could see how a blend could give much of the benefits of a synthetic with lower cost. I could also see how a company might sell a blend as a way of attracting folks like us that are willing to embrace the technology of a synthetic oil, but shy away from the cost of a pure synthetic like Mobil 1. In fact, that is how I’ve seen some of the blends marketed; as a value-priced synthetic equivalent. I really don’t know which is closer to the truth. In general, especially with a proper oil change frequency, I would think a proper dino oil (correct grade and viscosity range) would work fine.

WHAT I USE    
I use 4 quarts of Castrol GTX 10W-30 with 1 quart of Castrol Syntec 5W-50 and change the oil and filter every 3,500 miles. I like the clinging properties of the synthetic, but don’t feel it’s necessary to use 5 quarts of synthetic to gain it’s benefits. And I use Castrol because our local factory rep generously supports our car show!