Hi
As mentioned above the spec is WSS-M2C913-B and 5w30
theres plenty about like castrol edge or shell helix ultra or fuchs super syn oil are all great oils
shell is most probably the cheapest and if it were my diesel I'd change it twice a years, due to the issues of these engines. for peace of mind
also keep an eye on oil level, if it goes up, theres possibly diesel getting in there from either leaky injector or blocked dpf's, diesel in the engine oil, leads to snapped crank.
I use castrol edge professional 5w30 C1 in my S-type R, its had it from new, its not cheap, but my engine is still like new, never uses a drop of oil and its so quiet for a 18 year old car.
best having a look round for best prices, but try on ebay, opie oils, my-tyres, euro car parts have great deals as certain times.
cheers
Joe