Hi Simon,

I believe that you see a different frequency on changing from 33pF to 100pF.
But on using typical 5% capacitors the value should only change from 31.35 ... 34.65pF.

I must admit, that I never has compared oscillators vs crystals.
For communication (UART, CAN, Ethernet) the crystal was always good enough.
And for RTC applications I use radio control (DCF77) or let the user correct the deviation (write correction factor into EEPROM).

Peter