"say there was a zombie apocalypse and I was two quarts low…, could I throw some type f transmission fluid, or some dextron/mercron, in with the oil? "
Yup. It wouldn’t be good for the engine, but you’d have enough problems without worrying about your engine.
"could you run straight transmission fluid for oil?"
Yup. Once. But then your engine will be ruined and you won’t be able to get away from the apocalypse zombies.
Skip your son’s special oil. Use only what your owner’s manual recommends.
Okay, now to get serious. Automatic transmission fluid is not oil, it’s a type of hydraulic fluid.
The most critical job your oil performs is to maintain a proper pressurized fluid barrier between the crankshaft and connecting rods and their respective sleeve bearings. It does this by being pumped through channels in the parts and forced out the spaces between the surfaces and their bearings. It must have sufficient resistance to flow (viscosity) to NOT flow too easily through the spaces and to withstand the compression (squeeze) placed on it by the lateral (sideways) forces transferred to the parts from the combustion of the fuel in the cylinders. Motor oil is designed with this ability. Automatic transmission fluid does not. This resistance to being squeezed is every bit as important as the slipperiness of the oil. Note that what I write applies to the camshafts too, but the crank will seize from lack of lubrication long before the camshafts do, so IMHO it’s moot.
Automatic transmission fluid is designed to quickly and readily move through small orifices in the hydraulic solenoids and to transfer forces… quickly. ATF would easily flow out of the spaces between the crank & rods and their sleeve bearings, allowing loss of pressure and destruction of the surfaces that oil keeps separated.
In summary, motor oil is designed to NOT flow too easily, ATF is designed to flow very easily.
The things they have in common? They both remove heat and they both lubricate. But each in its own way.