Traditionally, base fluids are mineral or synthetic oils. Recently, there has been renewed interest in biological base stocks - vegetable oils and animal fats. Their biodegradable nature and nonpetroleum origins are two major reasons.
Because additives are reactive chemicals, they can interact with one another in a given performance package, either synergistically or antagonistically. The job of the formulator is to deliver intended performance by minimizing the antagonism and by maximizing the synergy, through careful balancing.
The performance package and the viscosity improver are usually sold separately. This is because the two are not always compatible when in concentrate form. When applications require a viscosity improver, it is blended in the base fluid, along with the performance package, to formulate the finished lubricant.
Lubricant additive suppliers develop general-purpose performance packages that meet industry specifications using widely available base stocks. The supplier may tailor them for the individual company's use.