I've always heard that it's not a good idea to switch between grades and brands and that you should always stick with what works. From to Scotty Kilmer to mechanics telling me my engine would have serious problems if i went from 5w40 to 10w40 because "clearances are used to 5w40" or whatever. In every car i owned i've switched between brands and grades almost every time it was due for an oil change based on what i have in my stash or what was on sale or simply out of curiosity and never had an oil leak appear or any kind of problem at all. Each time a problem such as a leak appeared is was from sitting for a long time. I don't know what this assumption is based on?