What is Inside a Motor Oil Bottle?

July 26, 2022 4 min read

All motor vehicles need lubrication – think of motor oils as the engine's lifeblood. But what are engine oils? Fundamentally, they are a combination of some base oils (mineral, synthetic, or both) and additives – specialty chemicals that give rise to specific desirable characteristics.

The above definition is an oversimplification – after all, a cake is just flour, eggs, and water, right? But just as there are thousands of cake recipes – some good, some bad – there are many different formulations (recipes) in the motor oil market.


Base Oil

Base oil provides the most crucial property of the oil – viscosity. The oil's viscosity (its "thickness") is the primary line of defense against engine wear. But not all base oils are created equal. There are many base oil types, but for engine oils it is helpful to characterize them by the Groups set out by the American Petroleum Institute (API).






Viscosity Index



<90 %


>0.03 %

80 to 120

· Lightly processed, crude-derived mineral oils

· Rarely seen in engine oils due to low VI

· Poor oxidation stability, but high solvency


>90 %


<0.03 %

80 to 120

· Hydrocracked, crude-derived mineral oils

· Typical of the “semi-syn” motor oils

· Good oxidation stability


>90 %


<0.03 %

>120, typically 120 – 140

· Heavily hydrocracked, crude-derived synthetic oils

· Typical of the “fully synthetic” motor oils

· Excellent oxidation stability


Polyalphaolefins (PAOs)

Typically 130 - 160

· Polymerized synthetic oils

· Typical of the “fully synthetic” high performance oils

· Excellent oxidation & thermal stability


All other oil types, including esters, white oils, vegetable oils, alkylated napthalenes, polyalkylene glycols, poly internal olefins, silicones, etc.

· Properties dependent on oil type

· Small volumes in synthetic engine oils for solvency

· Some niche motor oils are full esters


Over the years, the engine oil market has consolidated around mostly Group II and III base oils, which can be produced inexpensively and with a high degree of quality. The top-of-the-line PAOs will give ultimate performance, but the price premium generally sees them confined to high-performance racing oils.


Additives are like the "secret herbs and spices" of the lubricants world, imparting specific desirable properties to the motor oil, usually protecting either the lubricant or the engine against the wear and tear of extensive use in a hot, contaminated environment. The breadth of additives is only limited by the imagination of the formulator, but for our purposes, we will group them into "families" that perform similar functions.

Antiwear agents form sacrificial barrier films on metal surfaces to help protect them from wear. This task is usually assigned to the base oil, but internal combustion engines are so complex that the base oil viscosity is always a compromise. The antiwear agents afford an extra layer of protection in heavily loaded areas such as the cams and cylinder liners. The most common type is known as Zinc (ZDDP). The more aggressive forms of these compounds are known as extreme pressure (EP) additives – which can offer even more protection but need to react with the surface chemically. In both these additive types, there can be too much of a good thing – more Zinc can elevate friction inside the engine and increase ash levels, while high levels of EP can cause chemical corrosion, particularly in the soft metals found in the engine bearings and coolers.

Antiwear and EP agents are a subset of the friction modifiers, a group that includes polymers, organic Molybdenum compounds, and mechanical friction modifiers like graphite and Teflon. While these all function by slightly different mechanisms, the objective in a motor oil formulation is to form layers with low shear strength that allow metallic components to slide past each other easily.

Corrosion inhibitors also form sacrificial barrier films on metal surfaces to protect them from corrosion. Because the high-temperature environment of an engine usually produces acidic compounds and water that contribute to corrosion, the inhibitors exclude water from the surface. Assisting these molecules are the overbased detergents, which neutralize corrosion-causing acids and help clean deposits that build up in the engine over time. Once again, an excess of both these additive types can lead to seal breakdown, ash formation, and even corrosion problems, so these must be used judiciously.

A cousin of the detergents, dispersants perform a somewhat similar function in that they bind to contaminants such as deposits, sludge, and soot preventing them from sticking together to form larger abrasive particles. Dispersants are physically much larger than detergents, and an excess of these molecules can cause variations in low-temperature performance.

In many ways, dispersants wouldn't be required if the oil was free of contamination. One of the primary reasons that sludge and varnish form in the engine is the oxidation (cooking) of the oil. As frying oil left at high temperatures will eventually blacken and thicken, so will the motor oils. Antioxidants are amine and phenol-style molecules that seek out free radicals in the oil and prevent them from doing further damage, slowing down the aging process. If it weren't for antioxidants, oils would thicken dramatically over a short service life and cease to flow to critical areas of the engine.

This flow of oil becomes critical at very high and low temperatures. In an ideal world, we desire oils that stay thin at low temperatures to enable easy engine startup and remain thick at high temperatures to protect component surfaces. Mother nature gives the exact opposite – just as honey thickens in the fridge and becomes thin in a pan, so does oil. To counteract these affects, we have viscosity modifiers that improve the viscosity stability at high temperatures and pour point depressants that prevent oils from gelling at low temperatures.

While many lubricant manufacturers like to tout the amount of additive contained in their motor oils, the reality is that the skilled formulator uses as little additive as required to achieve the performance required in use. Once consumed, most of these additives become deposits that reduce engine performance. Additionally, complex chemical interactions occur between additives, so knowing how to leverage "synergies" between molecules is a crucial component of lubricant effectiveness. This is the strength of Torco's MPZ technology – a proprietary mix of additives proven to work together to reduce both engine wear and friction in our ultra-high-performance racing oils.