Hi Charlie; I've used Fantastic, or some other de-greasing detergents like dishwashing detergent, with a toothbrush and the gunk will be gone in about 15-20 seconds. Be sure that you thoroughly rinse off the chip in cold water and towell dry. Wait until the chip has thoroughly air dried (both sides) before putting it into a vinyl flip or holder. Any moisture trapped in the holder will cause problems for you later on no matter what cleaning method you use. I've had no ill-effects with this method... although I would be very reluctant to use it on hot stamp chips or the early paper inserts such as they used in "early" Deadwood.
I must caution you however, that it will not make your round edges square again..<g> .... like making brown eyes blue.... Cleaning only improves a chips appearance; it does not make a used chip new. Some chips that have been nicely cleaned I've seen offered as new. Happy collecting, Archie