18F1320 datasheet, DS39605F-page 12, says typical values are 33pf, and it also says that the values used are for design guidance only. And as it further states below that,

Different capacitor values may be required to produce
acceptable oscillator operation. The user should test
the performance of the oscillator over the expected
VDD and temperature range for the application

So, in short, it'll really depend on everything you do...how you mount everything, temperature range, actual voltage range of the PIC, phase of the moon
Testing is about the only way to verify anything.