Why do we change our engine oil?

PBO's engine whisperer Stu Davies ponders the not-so-silly question

- Stu Davies

Why do we change our engine oil? A seemingly innocuous question I was asked a few months ago.

Because we do? Because its dirty? Because the handbook says so? It got me to thinking and to do some research because it's not such a silly question!

Oil in our engine is there to lubricate, to prevent our engines from wearing, to cool it. Basically the moving parts of the engine 'float' on a thin film of oil which is pumped around the engine by a pump from the sump. This oil is also sprayed on the underside of the pistons to help cool them and to lubricate the piston rings and where they come into contact with the cylinder walls. To do all these things it has to remain stable under extreme pressure in the bearing areas, and it is also used in some engines to cool them. It also has to be able to negate acidic combustion products.