I think it is just aluminum colored paint! On my old car it did discolor and stain - but maybe I didn't take as good care of it as I should have....Turk cleaned his and then clear coated it - it stayed looking good. The Aviaid decal also discolors a lot.
I always thought they were aluminum too - surprised me the first time I picked one up - then I noticed how thin they are and realized that aluminum THAT thin would never work as an
oil pan.
Pat