I’m sure this is not new to many here and has been covered to various degrees in the past, but there are many newer members who wish to understand more about what is in a motor oil. To those I hope this may be helpful. To the professionals, chemists, and purists, don’t get too picky as this is written in layman terms to be understandable to the average guy.

Tom NJ

To a formulator, a motor oil is a complex blend of 10-15 ingredients carefully balanced and tested to meet the industry specifications and market claims. To a blender it can be as simple as mixing three liquids together and filling it into bottles. And to the consumer it is, for the most part, a mysterious golden fluid with confusing numbers and letters that all make the same claims about being the best product possible for your car. In reality, it is all of these things.

While some oil producers blend many individual components to make their motor oils, most oils are made by simply blending three fluids; a DI package, a VI improver, and a base oil. These fluids, however, are the complex products of extensive research and technology. Following is a brief summary of each:

DI Package

An acronym for Detergent Inhibitor package, this thick dark fluid is a concentrated cocktail containing most of the performance additives needed to formulate an oil. DI packs are generally made by additive companies, the largest of which are Lubrizol (independent), Oronite (a Chevron Texaco company), and Infineum (ExxonMobil/Shell joint venture). These companies have extensive R&D facilities with numerous engine test stands for developing and qualifying motor oil formulations against various global standards. The development and testing costs are so high that they are beyond the reach of many oil blenders and marketers, so the work is usually left to these experts to concoct the formulation and give it to their customers. Naturally the approvals (SM, CF etc.) are only valid if one follows the formula, which requires that you use their DI pack in approved base oils. Some majors develop their own proprietary additive systems and buy the components instead of the complete package.

The DI pack for an SM/CF passenger car motor oil is jam packed full of goodies as follows:

Dispersants: These are chemicals that can disperse and suspend solid particles formed in the combustion of fuel that might otherwise be deposited in your engine as sludge. Consisting mainly of polyamine chemistry, these molecules have “polar heads” that attach to acidic molecules and solids such as soot, and a hydrocarbon tail that keeps it all in suspension until removed by the filter or oil change. Think of them as pollywogs who surround a particle – the fat heads bite the particle and the tails keeps them swimming. Dispersants are the largest component in the DI pack, especially in diesel formulations where there are a lot more soot particles to deal with.

Detergents: Also polar in nature, these “organometalic” products made from organic chemicals and metals are responsible for neutralizing acids formed during the combustion process, and cleaning the engine from high temperature deposits by removing and preventing the adherence of deposit precursors. Some detergents are “overbased”, that is, forced to contain more metal atoms than they really want to, and are best at neutralizing acid by-products. Others are “neutral” detergents which are somewhat more effective at the cleaning process. The most common metal atoms used are Calcium (Ca), Magnesium (Mg), and Sodium (Na), and these are all measurable in the UOA and VOA analysis. The organic portions are usually sulfonates, phenates, and salicylates.

Friction Modifiers: Often esters or partial esters, these additives are very polar, thus attaching to metal surfaces to improve lubricity. FMs are used to improve fuel economy, as opposed to reducing wear, and are additive to the effects of lower viscosity.

Seal Conditioners: Also often esters, seal conditioners are potent additives used in small dosages and designed to keep sealspliable. These are especially important for highly paraffinic base oils such as Group IIIs or PAOs due to the tendency of these base oils to shrink and harden seals.

Zinc Dialkyldithiophosphate: Affectionately known as “ZDDP”, this miracle multi-purpose chemical and has been the chief anti-wear (AW), extreme pressure (EP), and anti-oxidant (AO) additive for decades. It is so effective and low cost that it is virtually irreplaceable, which is why it survives all efforts to remove phosphorus (P) from oils to protect the catalyst. With modern oils putting caps on the maximum P allowed, other additives are now being used to supplement this old standard, such as Molybdenum anti-wear compounds and ashless anti-oxidants. There are different types of ZDDPs including primaries, secondaries, and aryls, each with its own strengths & weaknesses, and the mix is balanced to the type of service the oil will see.

Anti-Oxidants: These sacrificial molecules react preferentially with oxygen to protect the other components from the degrading effects of oxidation. While oxygen is 21% of the air we breath, most people don’t realize that in its pure form it is so reactive it is considered a flammable gas! Even diluted in air, it is everywhere and wants to react with just about everything if conditions are right, such as high temperatures. Oxidation, the reaction with oxygen, is the main cause of oil thickening and left unchecked will lead to varnish and carbon deposits as well. With the ZDDP being reduced, supplemental AOs are more critical in modern oils and usually more than one kind is used to capitalize on the common synergistic properties they possess. The most common types are phenolics and amines.

Rust & Corrosion Inhibitors: These additives are smaller in dosage and are designed to protect iron alloys and yellow metals from corrosion induced by oxygen, acids and water. They work by attaching to metal surfaces and therefore compete with some other additives and base oils, so balance is critical.

Pour Point Depressants: These polymeric molecules interfere with the formation and growth of wax crystals from residual paraffins. They are generally not needed in full PAO and ester based oils since they contain no wax.

Anti-Foams: Often silicone products, these molecules are not soluble and work by suspending tiny micron sized droplets that prevent foam from forming or help the foam break faster.

Diluent Oil: Also called carrier oil, this component is usually mineral oil and is present at about 5-20% in the DI pack to solubilize all the additives and adjust the package to a consistent and manageable viscosity for pumping and blending.

Finished DI packages will vary in chemistry, balance, and dosage according to what kind of oil you are making. For example heavy duty diesel DIs will have more dispersants and be used at dosages up to about 15% of the finished oil. Passenger car/light truck DIs have less ZDDP and more anti-oxidants and are generally dosed at about 8-12%.

Viscosity Index Improvers

Abbreviated VIIs, these are huge polymeric molecules, often with molecular weights in the millions. Their purpose is to improve the viscosity index of the finished oil so that multi-grades can be made.

All organic liquids will thin out when heated and thicken up when cooled, but they don’t all do so at the same rate. Viscosity Index is simply a scale to compare the rate of viscosity change with temperature among different fluids. A fluid that thins more upon heating (and therefore thickens more upon cooling) has a lower VI than one that thins less and thickens less. Or put another way, higher VI oils change their viscosity less when the temperature changes. This can be a good property for lubricants that are used in a wide temperature range.

The VI scale was originally established by assigning a value of “0” (zero) to the worse known base oil at the time, and “100” to the best. The theory was that all other base oils would then fall between these end points. Apparently they didn’t anticipate synthetics or hydrocracked mineral oils back then.

The way VI Improvers work is that the huge molecules tend to coil up into balls when cold, thus having little effect on the oil’s flow (viscosity). When hot, however, the molecules uncoil and stretch out, thus interfering with the flow of the oil and causing an increase in viscosity (actually a reduction in thinning, but let’s not get technical). If you put these molecules into a light 5W base oil, the low temperature viscosity is little affected, i.e. remains a 5W, but the high temperature viscosity rises, giving for example a 5W-30 multi-grade. By reducing the thinning effect of heat, the Viscosity Index of the finished oil is increased.

VI Improvers are available an various chemistries and forms. Some are solids that need to be dissolved in the oil, but most are pre-dissolved in a carrier oil to give a thick, honey-like liquid that is easier to handle and faster to blend. Dosages are usually under 10% and vary with the VII chemistry, target oil grade, and base oil type.

People tend to think that the less VI Improver the better, but that depends on the type of VI Improver used. Some are much more shear stable than others, and a higher quantity of a shear stable VII may be better than a lower quantity of a non-shear stable VII. In addition to permanent viscosity loss cause by breaking (shearing) the large VII molecules, they also exhibit temporary viscosity losses under high shear, and this lowers the HTHS viscosity and improves fuel economy.

Base Oils

Constituting 80-90% of the finished motor oil, the base oil(s) play a very important role. The structure and stability of the base oils dictate the flow characteristics of the oil and the temperature range in which it can operate, as well as many other vital properties such as volatility, lubricity, and cleanliness. The two major categories of base oils are Mineral Oils and Synthetics.

Mineral oils begin with crude oil, a mixture of literally hundreds of different molecules derived from the decomposition of prehistoric plant and animal life. The lighter more volatile components of crude oil are stripped away to make gasoline and other fuels, and the heaviest components are used in asphalt and tar. It’s the middle cuts that have the right thickness or viscosity for lubricants, but first they must be cleaned up; undesirable components such as waxes, unsaturated hydrocarbons, and nitrogen and sulfur compounds must be removed. Modern processing techniques do a pretty good job of removing these undesirable components, good enough for well over 90% of the world’s lubricant applications, but they cannot remove all of the bad actors. And it’s these residual “weak links” that limit the capabilities of mineral oils, usually by triggering breakdown reactions at high temperatures or freezing up when cold. These inherent weaknesses limit the temperature range in which mineral oils can be used and shorten the useful life of the finished lubricant.

Mineral oils are further subdivided into three subgroups (Group I, Group II, Group III) that differ by the degree of processing they undergo. Higher groups have been subjected to hydrotreating or cracking to open aromatic (ringed) molecules, eliminate unstable double bonds, and remove other undesirables. This extra treating yields water-white clear liquid with higher VIs, enhanced oxidative stability, and lower volatility.

Group IIIs are a somewhat controversial class as they are derived from crude oil like Groups I & II, but their molecules have been so changed by severe processing that they are marketed as Synthetics. Most people now accept Group IIIs as synthetic, but the discussion remains heated among purists, and I’m going to duck by not taking a side here.

Synthetic base oils are manufactured by man from relatively pure and simple chemical building blocks, which are then reacted together or synthesized into new, larger molecules. The resulting synthetic basestock consists only of the preselected molecules and has no undesirable weak links that inhibit performance. This ability to preselect or design specific ideal molecules tailored for a given job, and then create those molecules and only those molecules, opens a whole new world for making superior basestocks for lubricants. In fact, the entire formulation approach is different: instead of trying to clean up a naturally occurring chemical soup to acceptable levels with a constant eye on cost, the synthetic chemist is able to focus on optimum performance in a specific application with the knowledge that he can build the necessary molecules to achieve it. And since full synthetic oils are generally a company’s premier offering, their best foot forward so to speak, the additives are often better and in higher doses as performance trumps cost.

In general, synthetic base oils offer higher oxidative and thermal stability, lower pour points, lower volatility, higher VI, higher flash points, higher lubricity, better fuel economy, and better engine cleanliness. The amount and balance of these improvements vary by synthetic type, and can be quite significant for the engine and user.

There are many types of synthetic base oils, the most common being Polyalphaolefins (PAOs), Esters, Alkylated Naphthenes (ANs), and more recently Group IIIs. These different types of synthetic base oils are often blended together (or even with mineral oils), to give the balance of properties desired. All offer improved performance, but at a higher price, which brings up the question of value – how much performance to you need, and how much should you pay for it?

For the average car owner, driving conditions are mild enough for conventional mineral oils to work satisfactorily, provided they are changed relatively frequently (3,000-5,000 miles). For those users with high performance engines, severe climates, hard driving, or utilizing long drain intervals, synthetics can offer good value and may even be required. And then there are those who so love their cars that nothing but the very best will do for their baby.

So, as you can see, modern motor oils are very simple mixtures of very complex ingredients. Choosing the right components of the right chemistry in the right dosages is a real balancing act, as each of the components have their own pluses and minuses and can interact or compete with each other. Don’t try this at home – leave it to companies you trust who have the technology, R&D, and resources to achieve the necessary balance so critical to performance.