Viscosity is the resistance of fluids against flowing. An oil film is formed between the two surfaces under pressure with viscosity and system parts are provided to work for long years without abrasion.
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.
As a general principle, mixing of oils with different properties and different brands is not appropriate. However, the decrease in oil was a variety of reasons and if it is necessary to complete the same viscosity and quality class can be added with other brand oils. It is recommended that the oil is completely drained and replaced with new oil at the most convenient time after the addition with another brand of oil.
Hydraulic brake fluid at the same quality level can be mixed with each other regardless of brand.
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.
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.
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.