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S100 is good, but you have to be careful with it. If you apply it heavy in some areas, they will end up glossy. I haven't tried Pig Spit, but I'm told it doesn't have that problem. I also like Adam's Undercarriage Spray. It's not meant for this purpose, but it holds up to the engine heat and doesn't develop glossy areas no matter how heavy you apply it.
I can't seem to find pig spit so I use S-100 and it works great. My engine was turning white in places. After applying the S-100 it looked good as new.
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