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.
