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Any time the engine is turning, you have the potential for wear. The most severe wear on most engines occurs at initial start up while the system is building up pressure. That's called dry starts. The engine creates friction at all times while it is running and the oil is the barrier to that. Oil deteriorates with time. It get contaminated with metals and chemicals from the combustion process. Also when oil is superheated, it turns to ash which when mixed with condensations forms sludge. This sludge can create barriers to good oil circulation. That's why it is vital to change oil on a frequent basis. It's probably the one thing that has the most bearing on how long that engine lasts. Synthetic oils have the ability to be more durable than petrolium oils, and so can be left in a little longer. The cost difference balances that ou though.
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That's the whole thing in a nut shell. Newer engines use lighter oil and quicker responding oil pumps to help with that.
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