I'm just ordering 12L case for wife's and my car. Mine is diesel that requires b4 and wife's is petrol a3 oil car. I would like to buy one oil to cover both cars. Looking at specs Ultra looks little better but maybe im not reading it correctly. Both are MB 229.5 approved and both meet FIAT 9.55535-z2 specs. I find that good enough, but diesel version of oil is only b4 without a3 specification. What should I choose if it makes any difference?