I did a little searching and found little on what additives MIGHT possibly clash or what that even means. There are a lot of mad scientist-types here, and it is tempting to blend oils or weights to get some superior product. I wonder about additive clash in general when I top-off with an other than crankcase oil. Non-detergent is not the way to go, you want to replenish additives and usually viscosity too -this to me means a diesel oil. Will the additives clash? Can additives clash enough to cause some chain-reaction resulting in a harmfull third chemical? Shell says no. They claim all their oils are mixable syn-to-dino and are tested to be compatible with competitors oils. http://www.shell.ca/code/products/commercial/lubricants/essentials/synthetic_nonsynthetic.html Who knows of a possible interaction between oil additives? Maybe this issue is overblown. My opinion is add whatever oil you want for topping, but don't mix your inital fill (other than mixing weights) not because of *clash*, but because LACK of correct amount of an important additive might render the remaining additive useless. Esters as an EP additive is one example, low concentrations of moly are useless too. I am mixing weights to use up some old SJ Superflo Synthetic, but will not mix brands.