"If the on-chip amplifier is a simple inverter, such as in the 8051, the user can select values for CX1 and CX2
between some 20 and 100 pF, depending on whether
start-up time or frequency stability is the more critical parameter in a specific application. If the on-chip amplifier is a Schmitt Trigger, such as in the 8048, smaller values of CX1 must be used (5 to 30 pF), in order to prevent the oscillator from running in a relaxation mode. "

The above should throw some light on the issue - it is from the Intel application note AP-155 which deals in detail about the Oscillators for MCU. Get the PDF direct from Intel site.

Raghu