The term "additive clash" is quite a broad description that could encompass myriad possible outcomes. It might be an unwanted interaction between detergent types (for example salicylates and sulfonates together can cause issues if not done well), it might be the combination of large numbers of surface-active ingredients (like antiwear, FMs, detergents, metal passivators etc) that get in each others way and prevent good performance, it might be the combination of different polymers (VM, PPD etc) that cause unwanted cold temperature effects, or it might be any other unwanted interaction between active ingredients.
There's no way of knowing what is in any specific oil and what it might or might not do when mixed with another, especially in the real world. By this I mean that it is possible to run compatibility tests between oils (although nobody has run every combination of every oil in every ratio at every temperature), but this would be in a clean lab, in clean glassware, at prescribed ratios and temperatures. Throw in real world conditions - moisture, fuel, engine parts, wear metals, combustion products, heat cycles etc and there's no way to predict what will happen.
Luckily most oils use additives of similar chemistries - the industry has sort of ended up in the same pool of resources and so major 'clashes' between oils is rare (the early days of synthetic oil pioneering did cause some issues and led to the enduring myth that synthetics and minerals shouldn't be mixed). As has been stated upthread, the likely outcome is not disastrous but maybe just sub-optimal, but in a way you may never notice.
There's no way of knowing what is in any specific oil and what it might or might not do when mixed with another, especially in the real world. By this I mean that it is possible to run compatibility tests between oils (although nobody has run every combination of every oil in every ratio at every temperature), but this would be in a clean lab, in clean glassware, at prescribed ratios and temperatures. Throw in real world conditions - moisture, fuel, engine parts, wear metals, combustion products, heat cycles etc and there's no way to predict what will happen.
Luckily most oils use additives of similar chemistries - the industry has sort of ended up in the same pool of resources and so major 'clashes' between oils is rare (the early days of synthetic oil pioneering did cause some issues and led to the enduring myth that synthetics and minerals shouldn't be mixed). As has been stated upthread, the likely outcome is not disastrous but maybe just sub-optimal, but in a way you may never notice.