High-quality lubricating oils are provided for gear units and worm gear units, either using mineral oils or synthetic oils which depends on the application. The high specific load in modern gear units sets high standards for the quality of the gear oils. By means of suitable oil additives a high load capacity of the gears against damages caused by scuffing, micro-pitting and pitting on the tooth flank is achieved. Furthermore, the additives have an influence on the ageing resistance and the foam characteristics of the oil. Excessive foaming has a negative impact on the load capacity of the gears and bearings. An excessive foaming of gear oils can also cause oil leakages on shaft seals and bleeders. Oil manufacturers normally test the foam characteristics of oils according to the ANSI/ASTM D 892 method or the DIN 51566 method based on it, by which air is blown into an oil sample for a specified time. As the results of this test often do not correspond to the foam characteristics watched in gear units a more practical-based test facility has been developed. With the foam tester a pair of gears rotates in the oil to be tested thus mixing an oil-air dispersion. The surface foam, the change in volume of the oil-air dispersion as well as its reduction are checked under circumstances that exist in gear units.