Motor Oil, Why

Our last post focused on what motor oil is and what the numbers on the bottle mean. Now, we bridge the gap to the whys of motor oil: Why use motor oil? Why should motor oil be changed?

Why use motor oil?
The main function of motor oil is to lubricate the moving parts of an internal combustion engine, that is to keep metal from directly rubbing on other metal. But in addition to this, motor oil also cools and cleans the engine. By reducing friction, motor oil keeps the engine cool, but it also internally cools the engine as it circulates through the oiling system. The cleaning action is evidenced by looking at used oil, as compared to new  oil, and seeing all of the visible suspended contaminates – it also contains contaminants that are not readily visible.

Why should the motor oil be changed?
The typical question is “when should I change my oil?” Well, when is more tied the the trivia of what. The actual question is “why should I change my motor oil?”

A review of the secondary functions of motor oil tell us that it acts as a coolant. Over time, exposure to the extreme heat present inside an internal combustion engine breaks down the oil molecules, reducing their efficiency in performing the primary function of lubricating the engine.

The other secondary function of motor oil is to clean the engine. The accumulation of contaminants also impedes the ability of the oil to perform its primary function of lubricating the engine. Not only is the oil trying to keep metal from rubbing on metal, but it also trying to keep its accumulated contaminants from rubbing on metal.

So, why should I change my motor oil? I should change it because it has become dirty and its molecules have broken down, inhibiting its ability to reduce friction, and thus it has become significantly less efficient at lubricating my engine.

How does this tie back to when should I change my motor oil? Well, I should change it when it has become dirty and its molecules have broken down, inhibiting its ability to reduce friction, and thus it has become significantly less efficient at lubricating my engine.

As indicated over at Edmunds.com, and many other places, in the past oil changes were recommended every 3,000 miles or 3 months – whichever came first. More recently intervals have been stretched as far as 15,000 miles or 1 year, using appropriate oil and filtration.

Historically, the 3,000-mile/3-month interval was a safe bet so as to remove the oil from your system before it became too contaminated or too broken down. The more recent longer intervals have been facilitated by newer vehicles with cleaner running engines, with tighter tolerances (slower contaminant accumulation) combined with synthetic base oils that have a higher molecular stability (the molecules hold together for longer).

There are a few factors to consider with regard to the question of when to change your motor oil, but be sure follow the recommendations of your vehicle manufacturer if it under an engine warranty and you expect any problems even remotely related to the oiling system to be covered under this warranty.

In our next post, we’ll address the more mundane, and less controversial, how.

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