The not-so-wise vehicle owners think that they are saving money by using cheaper oils in their engine or by chasing oil change specials. Using oil that doesn't meet specs or not changing the oil at the right interval will cause the engine to wear, and to build up sludge and deposits that cause the engine to run less efficiently.
Motor oil is a lubricating fluid that protects metal engine parts from wear, dissipates heat, suspends system debris to carry it to the oil filter, protects the seals in the engine and to provide corrosion protection for the engine parts.
Motor oil is made up of a base oil, or base oils, and a number of key additives that are needed to protect the engine and the oil itself.
Conventional motor oil is made from mineral oils from what are referred to as Group I and Group II base stocks. These oils break down at high temperatures and have poor low temperature performance too.
Group III base oils are further processed beyond Group I and II through hydrocracking, additional vacuum distillation and dewaxing. Group III base stock is very close in performance to "true synthetic" Group IV base stocks but at a substantial cost savings. Group II are referred to as hydrocrack base stocks.
Group IV base stocks are referred to as synthetic hydrocarbon base oils or polyalphaolefin (PAO). PAO-based oils are the best-quality base oils available (and cost significantly more).
Motor oils are also made up of additives (15% to 30% of your oil is additives) to achieve specific technical needs to protect the oil's life and the engine's components. Here is a list of the key additives:
Cheap oils are cheap because less additives are used and lower cost additives are used.
You will see oil divided into three major categories:
Oil is further broken down into "weights" or viscosity. You are probably familiar with seeing designations like 10W30, 0W20, etc.
What does the "W" stand for? It stands for winter. Motor oils are formulated for a "variable" viscosity or multiple viscosity depending on the temperature they are being operated in.
This is based on the temperature the vehicle is in. As temperatures drop the viscosity increases. Obviously if the viscosity was too high an engine could not start in cold weather. A 10W30 can be 10 viscosity in winter, 30 viscosity in warm weather. Variable viscosity is achieve with additives referred to as viscosity modifiers.
The word viscosity has been used quite a bit, but what is viscosity? According to the smart people at Princeton viscosity is "Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. A fluid with large viscosity resists motion because its molecular makeup gives it a lot of internal friction. A fluid with low viscosity flows easily because its molecular makeup results in very little friction when it is in motion."
A lower viscosity will create less drag in the operation of the engine, with the trade-off of some less protection between moving metal parts in the engine and less heat transfer.
Your motor oil is constantly under attack from several different sides. Let's consider the following:
Obviously the oil is subject to very high temperatures which can degrade the oil to the point of becoming sludge-like, or "mist" and causing oil to be carried to the air intake system and then the cylinders. Different engines operate at different temperature ranges so using the oil specified for the engine is critical. When you see an engine that is GDI and has one, or even two, turbos you know that the engine subject to very high temperatures.
Motor oil heated beyond a certain temperature, will begin to degrade. Heat will change the viscosity of the motor oil. The thermal stability of motor oil is not related to its additive package but the quality of its refining process and this is why full synthetic oil is required for engines that run hot like turbo charged engines and GDI engines.
Highly refined base motor oils also need higher levels of anti-oxidant additives.
Oxidation is the most important form of chemical breakdown of motor oil and its additives. The chemicals in motor oil are continuously reacting with oxygen and produces acidic conditions. These conditions encourage corrosion, create deposits, varnish and sludge and cause changes in oil viscosity.
The worst way to use an engine is when it is used infrequently and only for short trips. The engine does not reach a temperature necessary to boil and evaporate water that accumulates in the system from condensation. Quality motor oils contain corrosion inhibitors such as ZDDP (Zinc Diethyl Dithiophosphate), Calcium and Barium Sulphonates.
Mechanically motor oils are degraded by a process called shearing. The component of the oil that is affected most by these shear forces is the viscosity improvers. These viscosity-improvers allow the manufacturer of the oil to create multi-grade oils suitable for a wider temperature range of operation. The end result of these shear forces is a decrease in the viscosity of the oil, as well as a decrease in the viscosity index.
When conventional motor oil (Group I and Group II) has sheared beyond a specific point it will not revert back to it’s base structure when it cools down. Multi-viscosity petroleum motor oils are more susceptible to shearing than straight weight petroleum motor oils. Synthetic oils are extremely shear resistant and less prone to oil breakdown.
Oil will eventually fail as it accumulates debris. Over time the detergent in the oil can break down and its ability to suspend debris will fail. Contamination sources include dirt, sand and dust from the air, soot, unburned fuel in the oil, water from condensation of the combustion process, wear metal particulates that the oil filter cannot trap and hold, corrosion by-products and additive elements that have degraded.
Soot is a by-product of combustion and blow-by. Soot can be highly abrasive and it can load up an oil filter causing it to go to by-pass.
Unburned fuel is a key contaminant because fuel is rarely completely burned. Unburned fuel can mix with the oil present in the cylinders. Operating an engine that is not sufficiently warmed up can increase combustion blow-by.
When motor oil is diluted with fuel, the effect is that the viscosity is lowered.
The simple answer is to only use the oil specified by the car maker and that will be found in your owner's manual, or an oil that exceeds those specifications.
We personally only use a full synthetic oil in our vehicles (Group III base) and we change the oil every 5,000 to 7,000 miles).
How did we come up with that mileage range? It is based on a report from a national oil analyzing firm. We were told when we sent a sample in at 10,000 miles on a national full synthetic oil that there were metals in the oil indicating wear.
They told us in writing that in their opinion, based on decades of analyzing motor oil, that even good quality full synthetic oil should be changed at about 7,000 miles.
We make it a point to listen to people who are experts in what they do.