> Does someone knows if the maximal frequency printed on a
> chip determines an ability of the oscilator circuit too?

It used to be the case, probably still is, that all 628s (for example)
come from the same stock

They are then tested and sorted for GUARANTEED operation at the
speed limits. A device in the 4MHz pile may run at 10MHz or even
15MHz, but IT IS guaranteed to run properly under all conditions at
4MHz. Some 20MHz devices may run at 25MHz, but probably not
under all conditions. On an individual basis you could get lucky, but
it's not something you'd want to expect from a production run using
such a PIC unless every unit was speed tested

Economics plays a part too. Operation speed is a factor in pricing,
so is demand for a particular speed. Parts that are capable of spec
operation at 10MHz and 20MHz might be re-branded as 4MHz to
satisfy a market

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.