Ordinary engine oil is a by-product of the refining process and becomes available whether the refiner wants it or not. As a lubricant, it has very little value at all until it is doctored with a group of additives, from which comes the viscosity and durability rating shown on the container.
As we all are aware, the basic raw material is a very viscous black goo, that has to be diluted with solvents at the refinery so that it can be processed.
Unfortunately, these solvents
are carried over with the lubricating oil fraction and are the major reason
for the rapid deterioration of the additives in cheap oil.
Within 1000 Km, your ordinary oil is not doing much of a job of lubricating your engine. At high temperatures, this oil carbonises rapidly and most of the black residue that drains out at an oil change is not engine wear, but burnt, deteriorated, oil that has carbonised itself into oblivion.
A $1.50/litre motor oil, no
matter what the brand name, should not be left in your engine for more than
100% recycled oil, selling for eighty eight cents per litre in the big retail stores, has hit the market. Note that even these oils have an SF/CC rating, which only goes to show how low these standards really are!
So that, in general, engine oil has not improved much in the last five years and may, in fact, be less durable than it used to be.
Here come the so-called 100% synthetics which carry a little disclaimer on their label: " not including carrier oil".
These products are known as Hydrogenated Esters (HE) and are little more than properly modified mineral oils, although they certainly perform much more adequately and are probably good for 24,000 Km between drain periods, with regular filter changes.
Fourth generation products, are now available, as used in the aircraft industry, where oil changes are uncommon, at least in jet planes.
If one can find a way of formulating a PAO (polymer) based product containing no mineral oil whatsoever, at an affordable price, then one has a fourth generation engine lubricant that can remain in an engine, until the engine is rebuilt.
Filtration of pure PAO lubricants is not challenging for the filter because no carbon is present, and the filter is doing what it should do, eliminating the odd metal particle.
If you have a new car and wish to comply entirely with your warranty, then your owner's manual calls for an oil change every 12,000 Km.
Changing a pure PAO product at this distance is major overkill, but costs only $85 on average and is therefore no more expensive than cheap oil changed every 4000 Km, particularly if the latter service is done at the dealership.
Another common objection to leaving oil in an engine for long periods of time is contamination from products of combustion.
In the case of mineral oil, one can actually form an emulsion with water, resulting in a beige coloured `mayonnaise' that is some times seen on oil filler caps.
By contrast, PAO based lubricants shrug off water and acids and will not form emulsions. Consequently, as soon as the engine lubricant reaches the boiling point of the condensables, PAO's reject them through the PCV valve and go back to doing their job of lubricating the engine, completely unaffected by diluents of any kind.
If we still have your attention, here's a more technical explanation:
Since different fractions of the crude have different boiling points as well as different viscosities, progressive boiling is used. Those fractions with lower boiling points are allowed to vaporize, and are collected and then cooled.
These neutral fractions typically have lower viscosities, while the bright stocks (those with higher boiling points) generally have higher viscosities.
As such, we can separate oils by viscosity.
But here's a problem. If we compound an oil to have a relatively low viscosity (or a multi-vis oil with a significant amount of these lower boiling point/lower viscosity stocks) some of them will vaporize at high temperatures, resulting in higher oil consumption. What's left behind has a higher viscosity. Varnish and sludge are also present. If the decrease in viscosity, amount of sludge, varnish, and cam lobe wear are too high, it fails the API service test.
That's why a 5W-30 oil that meets the SF rating represents a major step. Those oils are said to be "energy saving" since their lower viscosity at lower temperatures (with thick-film lubrication. Remember, if the viscosity is too low, surface-to-surface contact may occur resulting in increased friction and wear!) results in lower part-to-part friction. Yet by passing the SF rating, it shows that it's still pretty good.
Now, there are many things in the average motor oil than various refined fractions of crude. Included are various additives, such as anti-wear agents, extreme pressure (EP) additives, anti-rust agents, .
Most of these are self-explanitory. They are added to enhance the performance of an oil. The EP additives are put in to help the oil hold up between surfaces which feature high contact stresses such as those between the cam lobes and followers. Detergents and dispersants are put in to help remove dirt and sludge and hold it in suspension, until it's either removed in the filter, or the oil is changed.
Also included are various oil modifiers such as pour point depressants, viscosity index (VI) improvers, and seal swell agents.
Pour point depressants are added to inhibit wax crystal growth at low temperatures. This gives the oil better cold cranking performance.
VI improvers are designed to help an oil's viscosity/temperature performance. Remember that at high temperatures, an oil's viscosity drops. If it drops too low, we lose film thickness, and are in big trouble! The viscosity index (VI) is a measurement of how an oil's viscosity changes with temperature, compared to reference oils. The higher the number, the better. VI improvers are polymer compounds with interlocking structures (polymers are long chain molecules). Because these chains are interlocked, they don't move as easily at high temperatures and resist viscosity loss. Unfortunately, they don't necessarily contribute anything to lubricity, and in fact begin to wear out under shear stresses. As they wear, the oil's VI deteriorates, and we're left with the old VI improver, which has to be held in suspension. This is another reason to change your oil frequently! The VI improver's sensitivity to high shear stress is significant in that if the shear stress is high enough, the oil may experience either a temporary or permanent loss of viscosity!
Finally, an oil company may add various compounds which help protect the base stock, such as anti-foam agents, antioxidants, and metal deactivators. The antioxidants are important as they prevent the oil from reacting with oxygen at high temperatures and forming sludge, varnish, and lacquer.
So where do synthetics fit in? What are they? The term "synthesize" means to put together from small bits. Rather than separating crude into various fractions as is done with conventional oils, synthetic base stocks are made by reacting various organic chemicals together. For instance, if an acid an an alcohol are allowed to react, a compound known as an ester is produced. (As an aside, the aroma present in flowers is generally produced by an ester. Others include butter, lard, tallow, linseed, cottonseed, and olive oils - although I wouldn't substitute my favorite engine oil for any of them in my cooking, or viceversa!) Other synthetic hydrocarbon compounds are also suitable for lubricating oils, and manufacturers may blend two or more compounds together to arrive at suitable properties.
It should be noted that many additives are also made of synthesized compounds.
First, though, let's compare a conventional oil to a synthetic. A synthetic may require considerably less VI improver to have the same viscosity index. Remember that the VI improver wears out. Synthetic's are also more thermally stable.
Synthetic base stocks also have lower pour points - often below -50 degrees F, and require little or no pour point depressant. In contrast, bright stocks may stop pouring at 25-30 degrees F, and need it.
Still, synthetics are a bit more expensive, so compounding one to compete directly with a conventional oil may not make economic sense. That's why they are usually made to have superior properties. The extra performance is often worth the cost penalty.
For instance, synthetics can be compounded with very low pour points. This gives good cold-cranking performance. They may also be compounded with slightly lower viscosities at lower temperatures (while still meeting SAE specifications). This helps to reduce friction, and results in less wear, and better fuel economy.
Now the 5W-30 "energy saving" oils will do the same thing, but as we've discussed before, to lower the viscosity, these oils may be compounded with fractions which have a higher volatility. After a period of time, they begin to boil off or oxidize, leaving behind an oil of higher viscosity. Now, that same oil may meet API SF specifications, but a synthetic may remain stable for a LONGER period of time. (Esters exhibit excellent performance in the API test. Other compounds are very, very good also.) That means that longer drain intervals are possible.
A word on use. Some synthetic compounds are not compatible with conventional oils. However, most manufacturers, have recognized that one may add a quart of their product to someone else's, and have compounded them to be. To do otherwise would be to pass up their intended market! (As an aside, I try to avoid having to mix conventional oil, if I can help it. While they are also compounded to be compatible, the performance may not be the same when mixed together. It's ok in a pinch, but I don't make a habit of it.) Also, the lower friction resulting from the use of a synthetic lubricant makes them unsuitable for break-in.
To sum up, synthetics provide an excellent alternative to conventional
- especially if better performance is required.
It's your choice!