Do you think humans cannot sense things on a very small scale. Especially in a vibration box like a Corolla(or Matrix)?I have a few questions ...
- what is "drag"?
- how does it "set in"?
- how is that "set in" measured?
- why is 5k too much but 4k is "safe"?
First a language thing, since you seem to doubt the existence of "drag" completely. Drag in all of its senses, indicates an interfering, opposing "force" or action against something. Can be physical, or mental, monetary, psychological "opposition" to whatever is desired.
Oil literally exists to reduce friction between metals. That friction makes a lot of things. Noise, vibration, heat, decreased "ease" of motion.
It's not like it's fiction. It's friction between metals being reduced by an oil+additives. This continuously decreases during the use of the oil. And the slight increases in friction makes the engine struggle ever so slightly with old oil compared to a fresh fill.
It's just that validating and authenticating a slight increase in the coefficient of friction between surfaces in an entire engine with used motor oil requires considerable specialized equipment to test such that a regular person cannot validate it.
The specimen remains in the family's possession right now. It was bought new in 2007 and timely cared for on the essentials like oil and transmission fluid.