Hi Luca,

Found this:

PIC16F87XA Data Sheet page 148
http://ww1.microchip.com/downloads/e...Doc/39582b.pdf

TABLE 14-2: CAPACITOR SELECTION FOR CRYSTAL OSCILLATOR
8 MHz = 15 - 33 pF
20 MHz = 15 - 33 pF
Note 1:
Higher capacitance increases the stability of oscillator but also increases the start-up time.

Someone else can help with the "SERIN-SEROUT with the internal oscillator" part.

-Adam-