In the past I have successfully cleared a Honda car engine aluminum rocker cover using the following products:
PPG DX-533 Aluminum Cleaner
PPG DX-501 Aluminum Conditioner
PPG DAU-75/DXR-80 Acrylic Urethane Clear
You can download the technical data sheets on the above products from the PPG North America Automotive Refinish website
The rocker cover I cleared had heavy oxidation on it. Prior to the DX-533 Aluminum Cleaner, I did a brush sandblast using the extra fine version of Black Beauty grit. The extra fine version is just a powder and does not leave a noticeable blast profile. If you only have reasonably light oxidation, the DX-533 should clean it up well enough.