I have used a lot of produc†s on Al in good condition that did an excellent job of polishing - any of the products sold for that purpose work fine. HOWEVER, for badly oxidized/pitted Al I learned the method many years ago...
I did a lot of work on old english bikes which all had Al chaincases, timing covers, tranny cases, etc. I tried to polish them with simichrome, flitz, all the polishes but nothing worked at all. Then a guy told me the "trick."
Forget the polishes - they won't work (I had figured that part out), you use sandpaper! You start with the lightest grade wet or dry paper you can - 400 will usually be course enough, sanding in one direction, then continue up the numbers to whatever gloss-point you want - 600, 800, 1000, 1500, 2000. At 2000 you're almost at mirror finish and Al polish will make it that way. You can stop at any level that gives you the finish you are seeking.
But...be prepared to sweat a lot!