One of the duties of engine oil is to keep systems clean. A qualified engine oil incorporates soot particles in engine after separating them and prevent them from sticking on metal surfaces due to the detergent-dispersant additives in its content. It cleans soot particles adherent on metal surfaces with cleaning agent. Therefore, engine oil becomes dirty and its color turns black. If the motor oil is dirty, it is doing its job well.
The aim of lubrication is to decrease friction by separating two surfaces, to ease movement by lubricating surfaces, to make cooling, to minimize wear and to avoid corrosion by generating film on the surface.
The information on the oil packaging contains important clues about performance of the oil. There should be information about five issues on the packaging in general. These; brand, its contents (be mineral, synthetic or synthetic based), viscosity grade (such as 5W-30, 10W-40), international performances (such as API CK-4, ACEA C3), OEM performances (such as MB-Approval 228.51, Volvo VDS- 4, VW 504 00/507 00).
Synthetic based oils are oils produced with chemical methods.
They are produced to lubricate in applications where traditionally obtained mineral oils cannot be sufficient.
Synthetic-based oils provide superior protection at higher temperatures than minerals, easier flow and pumpability at low temperatures , more stable film strength under extreme pressure and longer drain interval in difficult operating conditions.
The Association of Automotive Engineers (SAE) classifies automotive gear oils according to their kinematic viscosity values at 100 °C and their pumpability at low temperatures.
For example, the SAE 80W-90 in an automotive gear oil '90' describes the kinematic viscosity of this gear oil at 100 °C. As this number increases oil thickens, if the number decreases oil thins. The kinematic viscosity of '90' gear oil at 100 °C is between 13.50 and 18.50 cST while the kinematic viscosity of '140' gear oil at 100 °C must be between 24.00 and 32.50 cST. The number to the left of W (Winter) determines the performance of gear oil at low temperatures. Pumping temperature of '80W' gear oil is -26 °C while pumping temperature of '75W' gear oil is -40 °C.
It is the API classification used for gear oils. Oil that contains additives to balance excessive pressure and pulsed load for hypoid gear type differentials running under heavy conditions and that fulfills MIL-L-2105 specifications. In GL-5, there are more overpressure additives and it is generally used under heavy work conditions. Unless otherwise specified, GL-5 should not be used in places where GL-4 is recommended. Because the sulphide in overpressure additive which is present excessively in GL-5 corrodes yellow metals.