Paint stripper is another method. I wouldn't leave it overnight, it etches the aluminium.
Metho is fine as well, but once again it can etch polished alloy if left overnight.
My personal favourite is the crockpot method and I have never had engines go dark but it can require up to 48 hours depending on how bad the engine is.
Sometimes the soap method can darken aluminium as well, it depends on which soap I guess.