Microchip always specifies the capacitor values as (reference only).

Final capacitor selection requires a bit of testing as outlined in this Application Note ...

AN949 - Making Your Oscillator Work
http://ww1.microchip.com/downloads/e...tes/00949a.pdf

Here's the first two paragraphs ...
Quote Originally Posted by AN949
INTRODUCTION

Many Microchip customers ask us for help when
designing their oscillator circuits. Due to the great number
of combinations of VDD and temperature ranges,
crystal or resonator, loading capacitors, circuit layout
and circuit board materials, it is not practical to provide
a cookbook process that would work well for everyone
or even for most users. However, there are measures
that can be used to develop and test for good working
oscillator circuits.

This application note breaks down the oscillator design
process into the following three parts: specifying the
operating conditions, selecting the components and
testing oscillator performance. Other concerns are also
discussed that may be helpful during layout and
testing.