I asked this on the stackexchange automotive site but didn't get much response perhaps because it entails some chemistry knowledge. I could have asked without mentioning the automotive details however I think it puts the question into context making it easier to give a good answer.
When you drain an engine of oil in some engines about 12.5% of old oil remains because it coats engine parts, sits in galleys, and some is at the bottom of the sump container(below the drain plug) and won't come out unless you tilt the car which is quite impractical.
The general approach is to remove the older oil by dilution\further oil changes. As an example if you have a 4 liter capacity and an oil change allows you to fill 3.5l new oil, this means 0.5l\12.5% of old oil remains and so a further oil change will reduce the initial 12.5% to 1.5% and a further oil change to 0.19%.
However from my understanding this won't work when you mix oils of different viscosity because oils of different viscosity do not mix when they settle at cold\low temperature(which is where it will be at point of drain). But they do mix when they are at hot\engine temperature. Is this correct? Do I need to be using the same viscosity oil to ensure dilution happens at point of drain e.g. 5w30 needs to be mixed with 5w30 but adding 5w40 to 5w30 won't get the dilution effect? What would happen if you added 10w40 to 5w30, would you get dilution at point of drain?
Also how do the oils settle in practice? would the thinner oil sit on top of the thicker oil? Therefore if you are trying to dilute with a thinner oil the thicker oil may settle below the drain plug and sit below thinner oils in galleys so it may not dilute\get drained with successive oil changes? Suppose I had a container with 0.5 liter of 5w30 and 3.5 liters of 5w40 would the latter sit below the former? What if you had 5w30 mixed with 10w40?
Also can factors besides viscosity e.g. additives in oil or any other factors affect dilution?
My issue is that I have 0.5 liters of black and thick looking oil in my engine(which I think is affecting performance) and I don't really know the viscosity of the oil they put in. I don't want to do countless and costly oil changes trying to get it out so approaching this in an educated way may help.