hello nati, theres many oils that can be used in in your 300 tdi, 10/30, 10/40, 15w/40 and 20w/50 (in hot weather) like CharlesY said its best to use a cheaper oil and change it more often to get rid of all the small metal deposits and carbon build up than to use a synthetic or semi synthetic oil and change it less frequent, all a synthetic oil is, is a oil with a higher flash point so it is burned at a higher temp. also take a old computer harddrive to bits and get the magnet out and stick it on the sump bolt this will attract any pieces of metal instead of them floating round in your oil, i use 10w/40 mineral in mine and change every 4,500 or when the dipstick is as black as fire back lol, any more info needed on oil just ask because i work for a oil company
, Castrol your paying for the name, packaging and advertising