Hi George,
All of your colors from both companies should be well shaken before adding them. However, motor oil is not one of those colors that matters when you add it. M.O. is very runny (liquidy) and you can add it before or after heating with the same results. Z is right though......some colors are very funny that way because they have added substances that will settle to the bottom. My personal list of these "bad boys" are as follows:
white, avocado, black, brown, golden, lime, pumpkinseed, chartreuse, green pumpkin, sweet potato, minnow silver, oxblood. I'm sure there are more, but these are the colors I use most often. They all need to be mixed thoroughly.
Chris