Motor oil products appear under many different brand names and at a wide range of price points. There are specialist and general purpose oils, as well as variants for two stroke and four stroke engines. The main types of motor oil, however, are as follows:
Synthetic oils are chemical compounds created artificially. In contrast to mineral oils, the chemical structure of synthetics is more uniform, giving them much higher thermal stability. This means that synthetics typically outperform mineral oils at both high (above 185 degrees F) and low (below 0 degrees F) operating temperatures.
Synthetic oils also tend to have a much higher base number retention than petroleum based oils. The base number is a measure of the lubricant’s ability to neutralize acid. A high base number means the lubricant is better able to resist acid, making it useable longer. For this reason, synthetic oils tend to last much longer than other oil types. As a result, oil drain intervals in vehicles that use synthetics tend to be longer than those that do not. Other advantages include higher film strength in some synthetics, a higher viscosity index and lower engine hydrocarbon emissions, among others. On the other hand, this tendency to last longer also means that synthetics are typically more expensive than mineral oils.
Part or semi-synthetic oil is a mixture of mineral and synthetic oils, designed to keep costs low while improving the viscosity and lubrication properties of mineral oil. They also provide protection for heavy loads and high temperatures. Blended oils are usually less volatile, meaning the evaporate less, resulting in higher fuel efficiency due to reduced oil loss. These types of oil are usually more expensive than conventional oils, although less expensive than the high-end full synthetic brands.
Viscosity is a liquid’s ability to resist flow. When exposed to heat, motor oil thins. When it cools, it thickens. Generally speaking, motor oil which is more resistant to thinning is considered superior for a car’s engine, as thicker oil tend to be a better sealant and also provides a better and longer-lasting film of lubrication, which helps to protect the car’s moving parts.
Before the development of synthetic oil, most vehicle engines used mineral oil, which is derived from crude oil as a product of the oil distillation process. Because it is simpler to produce than synthetic oil, mineral oil is cheaper, but it may not perform as well across such a wide range of conditions; for example, the presence of wax in mineral oils can lead to poor flow properties in cold weather conditions, and at high temperatures poor oxidation stability can lead to sludge and acid build-up. In addition, the viscosity of mineral oil tends to change significantly with the temperature, resulting in the base oil thinning dramatically at high temperatures.
This is mainly attributed to the chemical ‘impurity’ of petroleum-based lubricants. Mineral oils are composed of various chemical elements (molecules), each subject to its own evaporation, oxygenation and burn points. Put simply, the moment you start your car, petroleum based oils start changing. Though additives do help to control acid, maintain viscosity levels, and keep soot suspended, these are consumed steadily over time. As a result, car owners using a mineral oil are typically recommended to change their oil every 3,000 to 5,000 miles.
With increasing concerns about the environmental impact of drilling for fossil fuels, as well as refining and using them, along with the fear that these resources will run out, there has been a great deal of interest in developing biological alternatives to synthetic and mineral motor oils. In the near future, more vehicle engines may be lubricated using vegetable based oils made from grain, seed or nuts.