In 1973 a modified SAE 10W-30 multigrade heavy duty engine oil designated OMD-75 was introduced into a wide variety of British Army equipment in place of the single SAE 10 and SAE 30 grades previously used. Generally the introduction of the multigrade engine oil into a wide variety of different equipments has been successful. However, some problems with the use of a multigrade engine oil in certain diesel air-cooled engines has been encountered probably associated with high base stock volatility. A number of other problems are also described and examples given where multigrade engine oils have caused difficulties. OMD-75 has been successfully used in Allison transmission systems.The use of the “FZG” gear machine as a method of test for the shear stability of viscosity index improvers is described and results presented for a number of different types of polymers. The FZG machine was found to be more severe than a diesel-injector rig and no correlation seems to exist between the two different methods of test.