I stand corrected, although I'm sure that aluminum oxide is a bit different than pure aluminum.
AS3 is still better because of the silver... The only way they could improve would be micronized diamond or something.
From they're website on AA:
Made with micronized aluminum oxide and boron nitride.
Arctic Alumina uses a layered composite of three unique shapes and sizes of ceramic particles to maximize particle-to-particle contact area and thermal transfer. This exclusive combination provides performance exceeding most metal based compounds.
Controlled triple phase viscosity.
Arctic Alumina does not contain any silicone. The suspension fluid is a proprietary mixture of advanced polysynthetic oils that work together to provide three distinctive functional phases. As it comes from the tube, Arctic Alumina's consistency is engineered for easy application in a thin even layer. During the CPU's initial use, the compound thins out to enhance the filling of the microscopic valleys and ensure the best physical contact between the heatsink and the CPU core. Then the compound thickens slightly over the next 50 to 200 hours of use to its final consistency designed for long-term stability.
(This should not be confused with conventional phase change pads that are pre-attached to many heatsinks. Those pads melt each time they get hot then re-solidify when they cool. The viscosity changes that Arctic Alumina goes through are much more subtle and ultimately much more effective.)
Absolute Stability.
Arctic Alumina will not separate, run, migrate, or bleed.
Electrical Insulator.
Arctic Alumina Compound is a pure electrical insulator, neither electrically conductive nor capacitive.