I always learn a little more every time I read a string regardless of what I already know. I agree about Simple Green, but as cautioned, be careful on aluminum.
One cleaner I haven't heard mentioned that's about the best stuff I've ever used is Mirachem Cleaner/Degreaser. It removes tought stuff such as fuel stains and exhaust deposits. The good thing is it's not corrosive. If you want, use it for removing tar and cleaning tires and wheels. It doesn't damage paint. But the engine compartment is where it really does the job. Spray it on, let it foam up, then wipe it off. The stuff works!
Check out their web site:
http://www.mirachem.com/500_Foaming.htm