Hi SOTATOSA
10 bit A/D converters do work in 8 bit mode. Just set LEFT justified and the 8 MSBs are held in the ADRESH register and the 2 LSBs are held in the ADRESL register bits 6 and 7 which you ignore. ADRESH register contains the 8 bit result. I think this is how DEFINE ADC_BITS 8 works. Setting 8 bit conversion AND RIGHT justified would cause some confusion. Hope this helps.
Phil