| 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.