I'll have to agree with not using harsh cleaners, on an assembled engine. Most aluminum cleaners are either highly alkaline or acidic, and this can be absorbed by paper gaskets, and get into tiny joints, and unless properly cleaned or neutralized, it will cause you more problems than you want to deal with. About the only method is a very mild abrasive, and a lot of ' elbow grease'. Unless you keep up with the cleaned area, it will tarnish back to a grey color, it's what alloyed aluminum does best.