Hi, isn’t 40mhz the sugjested pic18f’s clock frequency or 20mhz like all other pic’s? Why don’t you sell the board with th 18f running as fast as the specs allow.
The 18F family can run up to 40MHz using the internal PLL 4x multiplier. Which means you need an external 10MHz oscillator. We are considering having boards made with 10MHz reasonators but there hasn’t been enough demand as of yet. I’ve also heard rumors that the 40MHz PLL operation is sometimes buggy. This is probably only extreme cases.
You are welcome to solder in your own 10MHz xtal/reasonator 
-Nathan