what oil is best some are saying semi synth some are saying fully synth

or doesnt it matter in a hdi ?
just orderd fuel filter& new housing & pollen filter oil fiter air-filter anything else ishouid add to the list ?
ive took the aircon pump off a while back and my mate got a none aircon belt for us but icarnt see one anywhere
anyone know where ican get a none aircon aux belt from for a 110 hdi ?
my parts place doesnt have them listed ihope idont have to put another air con pump back on -belt doesnt look bad but imight aswell get it changed if ican whilst doing the other bits
thanks inadvance