This is the last engine oil checklist issued by MGR and shows what was regarded as the minimum quality to use.
The most important parameter is the ACEA specification. Those given here are now twenty years old and have been uprated to C and maybe higher). So, providing your selected oil fits the ACEA spec set out here for the Diesel engine, the rest is personal choice and the depth of your wallet. As you say, this question is perhaps the one most frequently asked and because members have personal favourite brands/types, usually leaves the asker even more confused than before. For example, Tesco branded oils bearing the appropriate ACEA codes will certainly fulfill requirements. Beyond that, it's the icing on the cake.
TC