Used industrial oils are among the most frequently generated petroleum-based wastes in Poland, posing a threat to the natural environment. Oil changes in vehicles or industrial machinery generate significant amounts of this waste. According to regulations, used oil must be properly disposed of. One method of reusing oil is its microfiltration. The first objective of the article is to present the methodology for testing oil compliance with standards for cleanliness classes. The second objective is to present the results of oil testing using portable oil microfiltration stations and to demonstrate that repeated microfiltration extends the service life of oil before the next replacement. The article shows that by applying microfiltration, the kinematic viscosity of the oil decreased by 1.5 cSt after 480 hours of operation, whereas without microfiltration the viscosity decreased by 3.5 cSt. The oil cleanliness class according to NAS 1638 with microfiltration remained stable at 7–8, while without microfiltration the class rapidly increased from 7 to 12.