I have studied the manual and it doesn't have the text you showed, surely I have an older version. (if you would have a link)
http://melabs.com/resources/index.htm#Manuals
With this information I'm not quite sure about what to do except letting the define_clock at 3.
for a pic18f4550 @20mHz the rc clock [3] is a poor choice (see data sheet) if you are using the adcin command . for accuracy the device needs to sleep during the conversion according to the data sheet and adcin will not do that