Here's some information from Fox Electronics web site (FAQ) that should help
you (pertinent excerpts below):

http://foxonline.com/techfaqs.htm#a4


4. The load capacitance (CL) of my parallel resonant crystal is rated at
20pF. How do I calculate the value of the load capacitors used in my
parallel resonant oscillator circuit?

Use this formula to approximate the value of capacitors needed:

CL=((C1 x C2) / (C1 + C2)) + Cstray

Cstray is the stray capacitance in the circuit, typically 2-5pF. If the
oscillation frequency is high, the capacitor values should be increased to
lower the frequency. If the frequency is low, the capacitor values should be
decreased, thus raising the oscillation frequency. When CL =20pF, C1 and C2
will be approximately 27-33pF each, depending on the amount of stray
capacitance.




11. What happens if I operate a crystal over its maximum drive level spec?

Exceeding the maximum drive level (power dissipation) of a crystal can lead
to an increased rate of aging, Drive Level Dependency (DLD) problems,
increased number and intensity of activity dips that can stop oscillation
and, at very high drive levels, breakage of the crystal blank.


Best regards,

Ken Pergola

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