Gear lubricants consisting of straight mineral oils will mix in all proportions at normal ambient temperatures. The resulting physical characteristics of the mixtures will not be an exact proportional average but will approximate this.
On the other hand, gear lubricants containing additives may be changed either chemically or physically by mixing those of different compositions. For example, some EP additives have limited solubility in high V.I. oils. If then such additives are blended with naphthenic oils which will hold them in suspension and such blends in turn are mixed with high V.I. oils, a portion of the additive might settle out of the mixture. Also, if gear oil containing lead soap were mixed with such oil containing an active sulfur compound, a precipitate of lead sulfide might be formed. Compatibility of gear lubricants is of little concern in industrial service where either the life of the lubricant is long or, if additional oil is required in a gear case, it will likely be from the same source. However, in automotive equipment the necessity of compatibility of gear oils is important because vehicles may be serviced by distributers handling different brands of oil than that originally used in the gear cases. Recognizing the possibility of mixtures of gear oils from various sources U.S. Government agencies have the following requirement in most gear lubricant specifications: “the lubricant shall be compatible with each of the other lubricants qualified under this specification.”Commercial multi-purpose gear oils are almost universally compatible with each other.
