Use a slower crystal !
The WDT has nothing to do with crystal oscillator of the chip, it uses an independent clock (Usually inchip RC oscillator)thats why the tolerance is +- 30%
I don't think the WDT is the ideal element to use Mahmood. You're better using another timer process, supervisd of course overall by the WDT, this will allow the supervisory process to do more than just reset the CPU.
Good point, but it looks missy for if WDT timeout occur the reset pin of the chip would be activated internally and I would lose my current program settings.
I would like it to reset for precautionary purposes just in case the mechanical system came back to operation in few hours time for some reason, then my circuit would work normally without the need for a reset.

Mahmood