Oil Grades
The American Petroleum Institute (API) along with the Society of Automotive Engineers (SAE) are responsible for determining both test and grading standards for oil. The viscosity index can be listed as SAE 15W-40 or some other group of numbers. In contrast, the test standard the oil conforms to is listed with the API prefix-API CI-4/SL, for example.
The additives are what make each brand, not the oil. Motor oil is considered to be fungible, which means that it is so alike in composition that different brands can be mixed together without a problem. But most petroleum chemists will tell you they don't like to mix brands and models of oil. That is simply due to the additives becoming diluted, not because of the base oil product.
Most modern diesel engine oils are classified as multi-grade, which indicates the oil has the characteristics of two different viscosity indexes. Since the temperature of the engine and oil changes from startup to maximum operating load, a multi-grade oil is required. When oil is cold, its viscosity changes and it becomes thicker. When heated, the viscosity index number drops and the oil thins.
For example, a 15W-40 oil would flow with the characteristics of 15-weight when cold but will maintain the oil cushion properties of 40-weight when heated. Even though this is the accepted definition of a multi-grade, the product is a compromise and doesn't have the absolute flow characteristics of a single grade oil of either weight. Therefore, you can't buy a can of straight 15W and 40W oil and mix them together to create a multi-grade of your own.