Recently, Mobile oil changed the formulation of the popular Delvac 15W-40 diesel oil. It was using a Group II base stock as Chevron Delo 400 15W-40. Now it uses a mixture of Group II and Group I base stocks. Remember that Group II base stocks are more refined than Group I and has less wax and other impurities.
The link below is from the Mobile Website and it explains why a cheaper base stock is better for your car:
Here are some quotes from them:
The new formulation incorporates a mixture of high viscosity index Group II and low volatility Group I base stocks that feature superior performance in heavy-duty engine oil applications. These new base stocks, manufactured using proprietary ExxonMobil technology, are uniquely coupled with progressive additive chemistry to further enhance the position of Mobil Delvac 1300 Super 15W-40 as the industry's technological leader.
It is likely you will notice that the oil is darker than the previous formulation. Although oil color is simply a physical property, our deeper color is being largely driven by chemistry changes that deliver stronger performance.