i used it on an exhaust manifold a few years back. works pretty well, but it's slow as shit.
for cleaning cylinder heads, i use aircraft remover. stuff annihilates anything carbon-based including, but not limited to: paint, oil, grease, skin, apples, ferrets, red licorice and the non-metallic parts of vacuum cleaners. won't hurt the metal though.
for oxidized aluminum, i use diluted lye to etch the surface/eat the oxidation. cast aluminum stuff usually turns a dark grey (called smutting). you can de-smut it with dilute hydrochloric (muriatic) acid or nitric acid (harder to find, but works better and doesn't harm the AL, where as HCl will attack Al as well).
i got turned onto the lye trick with Al when i started anodizing some of my brackets/custom parts. these are the porsche caliper brackets and 3SGE manifold throttle cable bracket i did a couple weeks ago:
this is the throttle bracket fresh out of the dye bath:
it's really not that hard, just need some sulfuric acid and a battery charger.