Oils have a finite lifetime – they will eventually degrade and/or become contaminated, and will need to be changed. Lubricants consist of a base stock (mineral or synthetic) and an additive package. As oil works to lubricate a piece of machinery, the additives become depleted and deactivated, which eventually renders the oil inefficient.
Technical Bulletin 52 discusses how lubricants degrade, and what the mechanisms are for additive depletion and degradation, including: neutralisation, shear down, hydrolysis, thermal degradation, water washing, particle scrubbing, surface adsorption, rubbing contact, condensation settling, filtration, aggregate adsorption, evaporation and centrifugation.