Pic Microcontroller - Secondary Oscillator
The watch crystal is fine.

The resistor limits the power to the crystal. If you look at the datasheet you'll see maximum drive level is 1 &micro;W, for other crystals it's even 0.5 &micro;W. So I wouldn't decrease the value or you may destroy the crystal. The drive level is different for each manufacturer, so there's no solid rule for the resistor's value. I would follow Microchip's advice.

A larger value may look even safer, but then the power will be too low, and there's the chance that the oscillator won't start. At 20 M&Omega; that will definitely be the case.  

Isn't it possible that the M&Omega; resistors you saw were between output and input of the oscillator?