Mobil	Technical Bulletin
Mobil Oil Australia Limited
Automotive No. 863

WHY DO WE CHANGE OIL?

Some machinery, for example steam turbines in power stations, does not require oil changes, the original charge being usually expected to last for the life of the machine, which may be 20 years or more.

Why then does the oil in a motor car engine have to be changed at regular intervals, which may represent only about 50 hours running?

To answer this question, it is as well to state that except under very unusual circumstances, oil does not “wear out”, “break down” or otherwise deteriorate to such an extent that it needs to be replaced. What happens is that it becomes contaminated with water, acids, burnt and un-burnt fuel, carbon particles and sludge so that it can no longer provide the desired degree of protection for engine components. But, it will be argued, most modern vehicles have an oil filter. Why does this not remove the contaminants? The answer here is that a filter can only remove solid particles above a certain size. It cannot remove water, acids, or fuel dilution, all of which pass through the full-flow filter just as readily as the oil.

As we have already seen, the oil contains additives to combat the effects of these contaminates. But there is a limit to the amount of contamination that even the best oil can neutralize, and there comes a time when the only satisfactory procedure is to drain the oil and replenish the engine with a new charge. Thus there arises the necessity for regular oil changes.

The question should now be asked “How often should engine oil be changed?” Unfortunately there is no simple answer to this. From what we have already discussed, it will now be apparent that we change oil, not because it has deteriorated, but because it has become contaminated with various harmful substances, and the greater the rate at which these enter the oil, the sooner an oil change will be necessary.

The things that influence this include engine condition and method of operation. A vehicle that is used mainly for short distance stop-start running will require more frequent oil changes than one used for regular long distance traveling, and a worn engine with leaky piston rings will contaminate the oil quicker than a new engine in good mechanical condition. Thus it is not unusual to specify oil change periods in terms of numbers of miles or days, whichever comes first.

It should also be borne in mind that a high performance product like Mobil Oil Super can handle more contaminate than other products, and hence longer oil change periods can be justified.

As a final comment on this subject, it is worth mentioning that it is normal for oil to darken in service. This is not an indication that the oil has deteriorated. It shows that it is picking up its load of contaminates and keeping then in suspension, where they can do no harm, and where they can be removed from the engine when the oil is changed.