Why must the oil in vehicle be changed?
Wear particles
When the vehicle is being driven, the oil must work as a very thin (approx. 1/1000th mm) lubricating film and at a pressure of 10(9)N/M2.
As a comparison, this pressure is equivalent to the pressure when working in the ocean at a depth of 100 km.
Sometimes it happens that there is a breakdown in the oil film, due to unevenness in the teeth surface, too high a temperature or too low a speed. When this happens, wear occurs on the teeth material and the oil becomes contaminated with wear particles.
Normally the oil is filtered through the oil filter, but it is not possible for the filter within a reasonable time to remove these particles which are as small as the oil film is thick.
The particles therefore contribute to wear.
The only way to remove these particles is to change the oil. This is very important after running-in, as the oil then contains considerable particle contamination.
The oil filter should always be changed in conjunction with an oil change.
