I've read that different viscosity of oils do not mix. If this is the case how do the oils get layered e.g. does the thinner oil float to the top of the thicker oil?
I've calculated with the first oil change 12.5% of old oil remains in my engine, and with a second change it gets reduced to 1.5% etc. However this assumes the old oil mixes with the new. If the newer oils were of different viscosity would it cause the amount of old oil to come out to be less or more since they do not mix?