You are right!Bmw do use same oil in Diesel and petrol variants......Just check your manual.Make sure you get the longlife oil as recommended in the Manual!With respect to brands,Mobil or Castrol,this is a matter of contract agreement between Bmw and the oil supplier!I have had Bmws M3 since 1988 up to now,and in the 80's and 90's Bmw recommended ONLY Mobil 1(5w 50 for my M3),since then it seems that Castrol gave BMW a better deal and thus they are using Castol Oils!I still prefer Mobil1 like a lot of people who have had Bmws for decades,I dont like Castrol because in my experience the oil has an unpleasant odour and it sends out more condensation through the oil breather pipe.......
My opinion!Stick to what it says on the Owners Manual on alternative oil grades and use Mobil1 that has the same specs.Btw the recommende viscosity is 5W 40...................
FYI Bmw use same oil for petrol and Diesel because the Diesels are Turbo and need an oil that can stand high temperatures,be able to lubricate the turbo parts that rotate at over 100,000 rpm,and will not deteriorate and block turbo oil lines!This oil needs to be synthetic based and has nothing to do with oils that are used in NORMAL diesel engines.