Qashqai wrote:On the other hand, I still don't understand why I have to change the oil every three months??? I understand the km/mile necessity, but why "three" months?
There is a simple reason why oil must be changed: To flush contaminants from the oil.
Water, gasoline, carbon, and metal deposits can accumulate in the oil. Sulphur is also present in the oil and when mixed with water under the heat and pressure of engine operation, it can form sulphuric acid. All of these can break down oil destroying it's lubricating properties and harm your engine.
Short trip city driving actually worsens oil contamination. Reason? Cold starts are when much of the contamination occurs. If only drive short distances, it's even MORE critical to have the engine oil changed at regular intervals.
Long distance driving (>25 min) is actually better for your engine. This allows the engine oil to warm up sufficiently so that some of the contaminants (water, fuel, etc) can be evaporated where it can be drawn into the engine to be burned. This is why Nissan suggests the longer interval under Schedule 2 if you do primarily highway driving.
So why the 3 month interval? Because the length of time those contaminants including water and acids stay in your engine, the more likely damage can occur. It has nothing to do with mileage!