Man, i didnt join this forum to have a petty argument, i cant be bothered explaining this s**t to you tyre kickers so i just coppied and pasted some simple, should have learnt when you were 10 information.


When motorists are asked: "Why does motor oil need to be changed?" the most common answers are:

It gets dirty

It breaks down

Because my mechanic told me


Amazingly all above reasons are WRONG !

And here is why:

In modern fuel injected electronically controlled engines with "sealed" crankcases there is no way that "ambient" dirt can get into the engine oil. Only if AIR FILTER is defective (torn or improperly installed or missing) or PISTON RINGS are excessively worn or broken, small amount of "dirt" can be introduced into the engine lubricating oil.
(For more info go to: Dirty Motor Oil in Q&A in Publications)

The Hydrocarbon Oil in its pure form is quite stable chemical, after all it has been in existence for millions of years before it was introduced into your vehicle. Almost no amount of pressure or mechanical stress will "damage" the oil molecules. Interaction with other chemicals, extreme heat and availability of oxygen will cause oxidation, formation of gum, varnish and sludge, but not a "Break Down". The long chain polymer molecules that are present in most multi-viscosity oils to give them high viscosity index, can however shear under stress and become shorter or smaller molecules. This loss of multi-viscosity property is sometimes referred to as "Viscosity Break-Down" a term that was introduced by CASTROL in their GTX Motor Oil commercials.

Unfortunately most mechanics are NOT Automotive Engineers, are NOT Lubrication Engineers, are NOT Tribologists, are NOT Chemists, are NOT Rheologists.
Neither is typical mechanic equipped with MICROSCOPE, VISCOMETER, TITRATION UNIT, SPECTRO-CHEMICAL ANALYZER.
One needs all of above to determine if any Oil needs to be changed. Also since such TEST can cost from $10 to $100, depending on accuracy and complexity, it is more convenient to change oil at some arbitrarily determined interval.





There are two reasons why lubricants need to be changed periodically:

The most important reason is chemical change of the lubricant itself. The lubricant looses its lubricating capacity due to chemical decomposition of the oil itself and due to depletion of the chemical additives that are present in modern oils. The more severe are the operating conditions, the faster does this change occur.

The second reason is removal of wear particles that are present in the lubricant and which can not be removed by normal filtration. The greater the loads to which the mechanism is subjected the faster is the wear and more wear particles are present in the oil. Frequent starting and stopping of the engine, especially in winter, also increases wear and formation of "cold sludge".

oh s**t, the oil I put in is runny like water.
(0W40)