In fact, engine manufacturers determine the oil change period in engines. Synthetic oils are oils that have more durability than mineral oils, so the oil service life is longer than mineral oils. The prolongation or reduction of the service period is closely related to the conditions of use and fuel quality, especially in diesel engines.

GL is the initials of the English words Gear and Lubricant, indicating gear oil. 1, 4 and 5 indicate performance levels according to the gear type. Example GL-1 is a mineral-based gear oil for spur gears. GL-5, on the other hand, indicates that it will be used in Hypoid type gears and its level..

W is the first letter of the English word Winter and indicates that the oil is suitable for winter conditions. 0 and 5 indicate the viscosity grade of the oil.

The performance level of that oil should be written on the engine oil packaging and labels. Performance values ​​are determined by organizations such as API and ACEA. According to API classification, S means gasoline engine oil, C means diesel engine oil, S/C indicates that oil can be used in both gasoline and diesel engines. The letter next to S and C indicates which year the oil is at the top performance level suitable for the engine technology. For example, SL: indicates the top performance level of 2001 in the gasoline class, CI-4 indicates the top performance level of 2002 in the diesel class

Synthetic oils are oils obtained by special chemical processes, while mineral oils are natural oils obtained by refining petroleum. It is not recommended to be mixed with each other.

Although there are applications made with observation in practice, in order to understand that a motor oil deteriorates and loses its function in real terms, it must be tested extensively in the laboratory.

The darkening of the color of the oil does not mean that the oil has deteriorated. One of the functions of oil is to keep the engine clean. Thanks to the detergent-dispersant additives it contains, it cleans the soot and pollution formed and takes it into its body. As a result, its color darkens. This shows that the oil is doing its job.

The decrease in the oil level with the burning of the oil entering the combustion chamber through the opening between the piston ring and piston is a sign of depletion. The reasons are as follows:
a-Not choosing an oil with the appropriate viscosity,
b-The oil escaping into the combustion chamber due to wear in the engine,
c-As a result of the constant use of the engine at high speed, the viscosity decreases with the effect of heat, and it burns by escaping into the combustion chamber
c- Decreases within the tolerance coming from the design of the engine.

Attention should be paid to the temperature of the engine.
Emptying and filling should be done while the vehicle is on level ground.
It should be ensured that the old oil has completely drained from the engine crankcase.
It should not be forgotten that the sump plug is closed again when the process is finished.