Hello,
I assume that the specification of 18pF is the shunt capacitance that should appear across the crystal in order to obtain the calibrated operating frequency. For the Pierce oscillator configuration, the shunt capacitance is the series combination of the capacitances to ground at each pin, including the input capacitance at the pins, and also any stray capacitance associated with the PCB traces. Assuming equal value capacitors, and allowing 10pF for input and stray capacitance at each pin, the value of each capacitor would be (18 * 2) - 10 = 26pF, say 27pF or thereabouts.
Any error will simply cause a frequency shift by a few kilohertz at most. So unless you are doing time keeping, this sort of error probably won't matter. In practice, capacitor values of 22pF or 33pF would also likely be satisfactory.
Regards,
Mac