So far I'm seeing some "phrases." Can anyone point to any information to back up these opinions? How about something from an oil company or additive manufacturer (like Lubrizol) stating that there are incompatibilities? Or maybe the American Petroleum Institute (The ones that do that API SG/CD stuff...)

In fact, my opinion (and it is only an opinion) is that the API would demand that all oils that meet the specification be fully interchangeable and have no adverse interactions if mixed.

Would I mix oils intentionally? No. Nor would I fret for a moment about mixing them.

There is no such requirement. API only tests to define, certify, and monitor engine oil performance deemed necessary for satisfactory equipment life and performance by vehicle and engine manufacturers.

http://www.api.org/certifications/engineoil/pubs/upload/1509_16thedition042007.pdf

In 2-3 years of owning my 300cd I have just figured this out. If I have the dipstick showing at half it doesn't leak or burn any more oil but closer to the full level it would. :o

If that were true, we'd never be able to switch oils. There's always some left-over oil after an oil change, especially in the oil cooler. If you look at some oil analyses, you'll see all oils are basically composed of the same elements in various concentrations. There are no magical or alien components.

They are compatible.

However, mixing two different oils will not necessarily give you all of the properties of either, so you could degrade the performance somewhat. Not enough to be a concern, but why not get the best protection and stick with one oil when practical.

Of all of my Mercedes, 103/104/602/603, I have never had one that consumed any significant amount of oil, and I always fill to the upper (full) line. Perhaps some of the older cars are different, but I have not found this to be true with these.

Same legend surrounds a Caterpillar engine of which I have owned 7, never found it to be true there either.