I've encountered several racers that have claimed to use Mobil 1 5W-20 synthetic oil in their transmissions, but have no idea what the long-term results were. That being said, engine oil does not have the proper friction modifiers and anti-foaming agents for use in an automatic transmission. I do not foresee the frictions and band(s) providing very good longevity or durability if you were to attempt to use engine oil in an automatic.
Also, as LS6 Tommy brought up, engine oil is not going to be very durable when encountering the sometimes extreme high temperatures that can be generated in the torque converter, especially if you are using a torque converter with a high stall speed.
If you're looking for something better than your average parts store ATF then I suggest using either Lucas Sure Shift or Lucas Marine ATF. We use these two products exclusively in everything we build. These fluids provide outstanding durability, longevity, and heat management properties. We use the Sure Shift in all street, street/strip, towing, and off-road applications. We use the Marine ATF in most extreme high horsepower applications (Sure Shift still works well in these also), although the Marine ATF can easily be used in lower-power applications as well.
Three good rules of thumb to follow when it comes to this type of thing:
1.) If it ain't broke, don't fix it.
2.) Don't try to re-invent the wheel.
3.) Anytime the conversation turns to "me and my buddies all do this...", stop listening. That is an official conversational red flag that the b.s. is really going to get thick if you allow the conversation to continue.