Motor oil: everything you always wanted to know

Mineral or synthetic? Monograde or multigrade?

Oil is the "blood" that circulates inside an engine, providing the necessary lubrication and cooling to all moving components that require it.
Typically, when we try to find out about automotive oil in general, manufacturers bombard us with a series of numbers, letters, codes, and strange properties that are difficult for anyone outside of chemistry to understand.
In this article, we'll try to explain in a simple way what motor oil is, how and why it's used, and its main properties.

Functions
The main function of oil is to prevent friction between two moving surfaces in contact, for example, between the cylinder walls and the pistons. If we observe them under a microscope, we can see that there are imperfections that, were it not for the oil, would end up generating excessive heat due to friction and would eventually fuse together, which is commonly known as "seizing." Thus, in addition to protecting the engine, lubricating it also prevents the energy it produces from being lost due to excessive friction, increasing engine efficiency and reducing internal wear.

Another function of oil is cooling. On its way through the engine, the oil absorbs heat from the parts it comes into contact with. It cools again when it returns to the main tank or passes through the radiator, reducing the risk of overheating.

The third function is cleaning. The oil must be able to dilute and remove carbon particles and other combustion residues, which are generally corrosive, thus preventing oxidation and the deterioration of internal parts. This is why a good oil must "dirty" itself; that is, it must perform its cleaning function, preventing residue buildup and, at the same time, keeping them in suspension, preventing them from settling in unwanted places.

Main properties
The classification of oils is based on two main characteristics: quality and viscosity.
The quality of the oil is determined by a wide variety of complex tests carried out by various organizations to determine its properties and resistance to deterioration during use. Two of the most important are:
-American Petroleum Institute (API)
-Japanese Automotive Standard Organization (JASO)
In the API classification for gasoline engines, the first letter is S, and the second letter indicates the oil's quality. They begin with SA, SB, SC... currently, the highest classifications are SJ, SL, SM, and SN (yes, they omit the letters I and K).

Oil viscosity is measured by the SAE (Society of Automotive Engineers) index and basically tells us how fluid an oil is: the lower the number, the more fluid, the higher the number, the thicker.
Most motorcycle oils today are multigrade, meaning they have two viscosity indexes: one for a cold engine (followed by a W) and another for an engine at operating temperature. For example, a 20W50 behaves at a viscosity index of 20 when the engine is cold and becomes less fluid, reaching a viscosity index of 50 as it warms up.

This facilitates starting and normal engine operation at low temperatures, while maintaining the oil at an ideal consistency when the engine is hot.
It's important to know that oil viscosity has nothing to do with its quality; it's merely an indicator of fluidity. The most appropriate index for each engine is determined by the manufacturer based on the different temperatures of the environment where the motorcycle is used. Keep in mind that too low a viscosity causes the lubrication system to lose pressure and protection, and too high a viscosity prevents the oil from properly reaching all the components where the engine is most exposed to wear, during start-up.

Types of oil
There are many types of oils, specific to their use in a motorcycle: fork oil, gearbox oil, engine oil, cardan oil, primary oil, etc. Each of these oils is designed with specific additives (anti-wear, detergents, and dispersants) to provide the right properties for the different functions to be performed, and it is essential to use the right oil for each case.

An engine with a separate clutch does not require the same additives as one with the clutch inside the engine compartment, and the gearbox oil is subject to very different pressure and temperature conditions than engine oil.
Regarding motor oils, we have mineral and synthetic oils.
Mineral oils come from petroleum and are produced from it through multiple processes in production plants and refineries. Crude oil has different components that make it suitable for different types of final products, so it always contains impurities or unwanted molecules.

Synthetic oil, on the other hand, is a product created from scratch in the laboratory, so it contains all the components necessary to perform its function and does not contain any unnecessary products.

Because their production takes longer and is more complex, they are more expensive than mineral oils, but in return they maintain their properties better during extreme situations, degrade less during use, and perform all the functions for which they are necessary (lubrication, cooling, and cleaning) better.
In theory, a mineral SJ oil has the same quality as a synthetic SJ oil, but the synthetic will degrade later and will better protect the engine from abuse or extreme temperatures (making an analogy, it's like cooking with sunflower oil or olive oil, the latter better withstands use in the kitchen...).

Conclusions
After reviewing the previous points, it only remains to emphasize the importance of always choosing the oil based on the engine manufacturer's recommendations and the ambient temperature range in which the motorcycle will be used. It is the engine's vital fluid and must be of the highest quality to withstand extreme operating conditions, protect its interior, and provide a smooth ride on the road. The same advice should be applied when choosing the oil filter.

One last comment: all engines are connected in some way to the outside world through engine vents.

During periods of inactivity, moisture-laden air can enter through these ducts, condensing inside the engine and mixing with the oil, degrading it and rusting internal parts.

When the engine reaches operating temperature, this moisture turns back into vapor and is expelled through the engine vents. Therefore, it is very important to shorten oil changes in situations where, after periods of inactivity, short journeys are made during which the engine does not fully warm up, or in conditions of high ambient humidity.

And if you're still unsure about which oil to use in your motorcycle, you can see a selection of the best motorcycle oils at this link .

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