Seriously speaking, I don't know exactly, how the PIC18F oscillator block would perform in a high accuracy oscillator. It's simply not specified for this purpose. In the usual enviroment, you can expect additional jitter caused by other digital chip activities that aren't present with a dedicated oscillator component. But jitter doesn't necessarily involve drift of the average frequency.
You should also asume, that Microchip is satisfied with basic crystal oscillator accuracy and don't implement special measures to reduce drift effects introduced by the oscillator circuits.