Are you using 12F1840 or the 12LF1840? Ifn I read the datasheet correctly you can achieve some significant power savings with the LF that you may not have to do the hokey pokey with the oscillator frequency. Though the LF is about 10% more cost if it gets you to your goal it may be worth it.

Is this a battery application or an experiment? Just curious why a few nA would be important.