Unless you make more than an auto oil engineer.?:?
You can certainly run motor oil or even ATF in them, but many people choose to use a synthetic gear oil because it last longer and resits breaking down. When the Bronco's where built, running motor oil in a T-Case was normal but that does not mean it is the best suited for the job. That being said, the Dana 20 is such a simple gear box that you can pretty much run anything you want in it, so long as it is thick enough to keep the friction and therefore heat down.
The main thing I would recommend is no matter what you choose run synthetic, it really is better oil. Also for you guys that are running detergent you really should swap it out for non-detergent, the detergent brittle's the yellow metal's in the T-Case, in the D20 that would be the two washers at the ends of the center gear shaft. Honestly with how the D20 is designed even if those two washer split due to hardening and corrosion. It probably would have little effect on the operation of the D20 it would probably just be noisy as hell. But it's best to not chance it, the detergent is not good for brass or bronze. The thing is, it takes a long time to erode it so you don't see the effect immediately.
