In any internal combustion engine, the indexes ofoil aging all change in the same general manner [1 3]. In the initial stage of operation, the concentrations of particulate matter, both organic and inorganic, increase very rapidly, as does the oil acid number; simultaneously, the base number drops off. It has been noted that the rate at which acidic product5 of aging are neutralized will decrease as the alkaline reserve of the oil becomes exhausted. Engine oil changes as a policy of preventive maintenance are based on operating-hour limits established by the engine manufacturer. However, engine service conditions differ from one application to another; therefore, any mandated oil change without quality evaluation may prove to be either premature (the oil is still good) or overdue (severe wear of engine parts has already occurred). A number of modern methods are available for evaluating used oil quality. However, any widespread use of these methods under the conditions existing in agricultural operations has been held back by the complexity of the laboratory equipment and the highly trained analysts that are required for such evaluations. Apart from laboratory test methods, rapid methods that have been developed can provide a rating of oil condition that is sufficiently accurate for practical purposes. One of the highly informative indexes of oil quality is the base number. After a rapid determination of base number has been performed, the value is compared with the maximum allowable limit, and a decision is made whether to replace the oil or extend its operation. At VIITiN, a simple, sufficiently accurate, rapid method has been developed for evaluating the base number of fresh and used motor oils. For this method, a 10-ml glass vial is half filled with distilled water, and a piece of indicator paper, prepared by a VIITiN method, is placed in the vial. Then 2 ml of an organic solvent is added, and finally one or two drops of the test oil sample. This mixture is shaken and then allowed to settle for 3 5 min. The color change of the solution is used to determine the base number of the oil. A yellow color of the solution indicates a base number below 1.5 mg KOH/g, green from 1.7 to 2.5, and blue above 3. For normal operation of a medium-performance engine, the fresh oil should have a base number of 3.5 4 mg KOH/g, and for a high-performance engine 6 6 . 5 mg KOH/g. Specific levels have been established for the drop of oil base number during the course of engine operation, at which the oil should be changed. For high-performance engines, this level of hase number is 2.5, and for medium-performance engines 1.5.1 Along with the commercialization of lube oil production and sales, cases have been encountered in which engines have failed as a result of low quality of the additives or an inadequate additive package. As a result of work performed at VIITiN in 1996 1997 on the analysis of the oils in current use and additional development of the rapid method for evaluating base number, the color of the indicator solution for fresh oils was rated on a standard scale as follows: