This is why I hate used vehicles with high miles. You have no idea where or what its been through. Did the owner care? Did they even use the proper fuel grade? I see BMW owners who use regular and laugh gleefully at those stupid TV analysis you see on news saying regular and premium have no different effects on engines; And say see, I told you, they are all the same.
Of course BMW colludes with oil companies to sell you premium.
Now why would BMW agree to it when it makes their cars more expensive to run? Also, these same people don't know some engines are high compression and some are lower compression, much less what the effect of compression ratios is.
Once they see this sort of crap in their engine, surely even an idiot can see there is no way it can be good. Guess what, they do what is only natural, they make it someone else's problem. I have never let my oil go 15K miles and never will. I keep my cars for a while and drive a lot. I have never leased.