Service at dealerships
I warned my service advisor that I will check the color of the engine oil after they claim it was changed. I take a white towel, and check the color.
If it looks anywhere near black, I refuse to accept. Same thing I do is with the brake fluid flush.
I do know that oil turns black after the car is driven a bit after oil and filter change. But I check it before I leave the dealership.
I found that once the oil looked black and may be they never changed it
or was partially changed. They changed the oil and looked a lot better.
I bring white paper towels.
I am a Chemical Engr. worked for ExxonMobil.