The earliest engine oil was a mixture of base oils only. The oil could only go for 100 miles. (160Km) before it is changed. If you are travelling to Kisumu or Eldoret from Nairobi, you would have to change oil in Nakuru and also change when you reach your destination. If you go further the oil would thicken and clog the filter and the oil ways and your engine could seize.
After many years of research a chemical was found that when added to the oil in a certain proportion, the oil will last for 400 miles. This means you could drive to Eldoret and back without changing oil on the way. This was a major achievement. Today, diesel engine oils in Europe and America can go for over 120,000Km without changing oil. This was perhaps the first additive and was called anti oxidant. It stops oxidation and therefore thickening of the oil.
An additive is therefore an oil soluble chemical which when added to an oil will enhance the performance of the oil.