Can someone please explain to me the difference between A3/B3 and A3/B4? Most "euro" oils contain both specifications, however, in looking at high-mileage oils, Castrol EDGE High Mileage 10W-40 is A3/B4 while Mobil 1 High Mileage 10W-40 is A3/B3. Is one specification better than the other? Thanks guys. Edit: I did search the forum, but couldn't find any definitive answer.