WHAT ENGINE OIL PLEASE

Hello all. I have a V 220 CDI, it has now covered nearly 5000 miles and the "ASSYST" oil check informes me that I need to add one litre of oil. Being of the generation that used a dip stick I confirmed that this is correct. After a visit to the garage I was totally confused with the vast array of oils on offer, synthetic, semi-synthetic, ordinary and so on. My question is, what is the "factory fitted" oil and what oil should I use for topping up from the major UK brands. Thanks in advance John