I have cleaned a few thousand Empress and PCA primaries and found that a bucket of warm water and Oxyclean knocked off 90%+ of the gunk leaving a little dirt in the crevices.
I would let them soak for s few hours and stir them up with a brush every so often. Not to bad at all.
The real time consuming part was laying them all out to dry and them oiling them. That is what made me want to buy only new chips from now on!!!!