I don't know the cause either but if you look at your oil, you can see it gradually turning colors, from pale ale to stout. At 5k in the US you're about at the latter, so I'm inclined to believe they're telling the truth.
> Oil changes color as it captures contaminates such as soot, carbon deposits, etc. It can operate effectively while brown-ish, but if the oil is carrying too much it will blacken and start to thicken at which point it will be much less effective at lubricating your engine.
> Oil changes color as it captures contaminates such as soot, carbon deposits, etc. It can operate effectively while brown-ish, but if the oil is carrying too much it will blacken and start to thicken at which point it will be much less effective at lubricating your engine.