I can't really distinguish 2 mils with a feeler gage, so sounds like cold adjustment is also acceptable. My guess is that the 10 & 20 mil specs were pulled out of the air by a Chrysler engineer, probably with Marketing saying "make it an even number". Had they used metric then, I bet they would have spec'ed "0.25 & 0.5 mm". If you adjust your valves annually (or is that "anal-ly"), you could probably get by with 6 mil & 12 mil, and your engine would perform better.
