Yeah that's what I was kinda worried about but I don't really understand it at all. I mean they're all oil based lubricants right? How can they not work (blend) together?
Other than different qualities or characteristics (viscosity, ideal performance temp, heat dissipation) don't they all do the same thing??
They do NOT all do the same thing.
It is not the blending as most oils will mix with each other and not all lubricants are the same. Just because one oil will mix with another doesn't say anything about actual compatibility and how it will affect the machinery.
ATF's and GL-5 gear oils are both lubricating fluids and will readily mix, but you would not logically want to mix ATF with GL-5 gear lube and put that mix in your differential. Viscosity and additive differences are just two of the issues.
Not all lubricants have the same additive chemistry which is why we use specific additive packages for specific applications in finished products.
Likewise, LSD additive, while it will mix with the ATF, is not suitable in a mechanical component speced for ATF because it will affect the total chemistry of the resulting mix, and not in a good way.