Can't speak for synthetic oils (never used them )but as far as conventional oils there had better not be a problem with adding one brand to another , I could see some serious legal issues if any brand of oil cannot be mixed with others. The API standards are supposed to be a uniform standard so that oils can conform to them (and are backwards compatible). If it says it is SL, it better be SL, and if it cannot be mixed with other oils there had BETTER be a VERY LARGE DISCLAIMER on the bottle. Adding that paper and metal can of oil that you found on the back of the bottom shelf marked SA to the new car that is running SL does not make sense, but adding the SL oil to top off the vehicle with SJ would only be slightly IMPROVING the quality, not degrading it. If the additives in one brand would clash with additives in another there would be some serious issues that we would have already heard about.
Think about all the folks that get the oil changed at the quickie places every 3k miles, of the folks I know that do it only some of them always go to the same place. The rest go wherever is having a sale on the service.
I believe that most of us on these posts really believe strongly that the brand that we use is the best, that's why we talk the way we do. But the vast majority of folks who drive cars probably don't know the brand or weight that is in the vehicle at the moment.
To sum it all up, I respectfully dis-agree that oil brands cannot or should not be mixed, I don't even have a problem with mixing weights. It seems the oil companies don't have a problem mixing synthetics with petroleums, and if you ask ANY of them, they all tell you that as long as it meets the standard it has to be compatible. I think most everyone does try to stay brand loyal, but if we stray, there's not going to be any little tap on the shoulder saying 'you're gonna pay for that.'
Now if FRAM came out with oil I probably would be a little worried ....
see y'all