Never add supplementary additives to quality engine oil
As a Fuels and Lubricants Engineer for a major oil company, I can categorically state to never add oil supplemental oil additives to your engine oil. Modern engine oil formulations are a complex set of base oils and additives that have been through a rigourous testing regime. The different additives within engine oil formulations have a chemical and performance effect on each other and the right balance is required for the oil to perform at its optimum. Supplemental additves can have a disastrous effect by upsetting this balance, and can result in additive drop out, oil thickening or diluting, and performance compromises. Often supplemental additives focus on one particular function, e.g. antiwear or viscosity, and this can be at the expense of another function such as anti-corrosion or dispersancy. Additionally, no oil manufacturer will warrant their product after it has been contaminted by supplemental oil additives. In summary, stick with the major and reputable engine oil manufacturers, adhere to oil drian intervals and alsways ensure the correct type of oil is used and you should never have an engine oil problem.