oven cleaner is caustic. which will instantly attack anything that is not ferrous ( iron/steel ) and soda balsting while being less aggressive, any of that abrasive media makes it into your oil, bad things happen. hydroflouric acid works a little less agressively than caustic, but will turn your case black. my 2 cents would be castrol super-clean on the case with a stiff brush ( also caustic, but less volatile) and have your fan shroud "peened" with #8 glass bead at around 40 psi, which will clean, but also seal up the porosity of the casting, but leave a nice sheen