my dad bought a 2001 volvo s80 2.9 from his dealership at work last year for 1000 it had 164K on it had CEL on. I Replaced all six coil's (OEM) and plugs. and changed the oil to Mobiil 1 0w40. I have gone threw most of the stickys could not find some info. also the timing belt has been replaced at 110k

1. What Octane is optimal for the 2.9 s80?
2. What is the most common problems i should look out for?
3. is there a way to fix the cd player?