Diff oil out the vent, why?

For longer higher speed use, the factory fill level is appropriate. I've heard that for 1/8 and 1/4 mile use etc, you can get away with around 1 quart of gear oil, as that will provide adequate lubrication, shack-load protection and require less power to spin. I have not tried this, but there are guys & gals on here with vast knowledge and experience that may chime in.