Aluminum Differential Cover Thermal Results

That picture doesn't show what is implied. Put a piece of styrofoam or other insulation on the cover, and a picture of the outside will read an even lower temperature even if the internal temperature is unchanged. Aluminum is about a 4 times better thermal conductor than steel, but you also have to consider the thickness. I'll bet that aluminum casting is more than 4 times as thick as the steel original, so probably provides less cooling.

GordonThree is right, 10 degrees lower for both ambient and the same spot on the diff. The cover isn't making a difference.