a 4 mhz one could be run {although it did hang every so often}
at 20 mhz using a xtal and a 7404 as its driver
then just using one pin on the pic set too hs mode and told to compile for 20 mhz
it ran ok so i guess it is like potatoes you buy....
You might get away with overclocking, but then again
you might not. Think of it this way - if the part was
stable at a higher frequency than specified, you can be
sure that the manufacturer is going to sell it as spec'd
for that higher frequency.