Hi,
for an RC oscillator the output frequency is proportional to 1/(R x C).
The higner the C the lower the frequency.
in other words:
C is proportional to period time.
For sure there will be some influence:
* circuit
* stray capacitance
* stray inductance
* thermal drift (could be cancelled out according Barry´s idea)
***
I usually use just a comparator with push-pull-output, three equal resitors (bias and hysteresis) and the "R" and "C" to determine the frequency.
.. and like always: the "C" for power supply decoupling.
Simple and fairly stable over time, temperature and over supply voltage.
I recommend to use a good CMOS rail-to-rail Comparator with push-pull-output.
Not an OPAMP like the LM124. .. it will degrade performance
Klaus