Alain,
Yep, you're right! I didn't remeber correctly..... :-(

To quote the docs:
If a 4MHz oscillator is used, the Period of the generated pulse will be in 10us increments. If a 20MHz oscillator is used, Period will have a 2us resolution. Defining an OSC value has no effect on PULSOUT. The resolution always changes with the actual oscillator speed.
Sorry for the confusion!

/Henrik.