For many years European farmers have used Tractor Oils Universal (TOU)-suitable for use in both engine and power train. Generally, these lubricants have been MIL-L-2104B engine oils containing sufficient additional zinc dithiophosphate to satisfy the mild extreme-pressure requirement of the power train. However, when such lubricants were used in the oil-immersed brakes introduced on tractors in recent times, experience showed that they were incapable of preventing ‘squawk’.A ‘new generation’ of universal tractor lubricants has been developed to overcome this problem. As well as providing acceptable anti-squawk performance in wet brakes, these lubricants-known as Super Tractor Oil Universal (STOU)-give improved performance in naturally aspirated and turbocharged diesel engines and in gasoline engines, and are suitable for use in fluid-immersed power take-off units.The history of universal lubricants is briefly reviewed, and the deficiencies of early STOU fluids are noted. The development of STOU additives for the latest STOU fluids is described with reference to tests in full-scale tractor equipment and in acknowledged test engines. The effect of changing the base stocks is mentioned, and the advantages of the new products are discussed in terms of performance in engines to the standards of the MIL-L-2104C and MIL-L-46152 specifications, and in transmissions and wet brakes to levels that satisfy the majority of tractor manufacturers' requirements.