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This paper outlines the second part in a series on the effect of polymeric additives commonly known as viscosity modifiers (VM) or viscosity index improvers (VII) on gear oil efficiency and durability. The main role of the VM is to improve cold temperature lubrication and reduce the rate of viscosity reduction as the gear oil warms to operating temperature. However, in addition to improved operating efficiency across a broad temperature range compared to monograde fluids the VM can impart a number of other significant rheological improvements to the fluid [1]. This paper expands on the first paper in the series [2], covering further aspects in fluid efficiency, the effect of VM chemistry on these and their relationship to differences in hypoid and spur gear rig efficiency testing. Numerous VM chemistry types are available and the VM chemistry and shear stability is key to fluid efficiency and durability.

Four 1998 Mitsubishi Carismas, two equipped with direct injection (GDI) and two with port fuel injection engines (PFI) were tested in a designed experiment to determine the effect of mileage accumulation cycle, engine type, fuel and lubricant type on engine wear and engine oil performance parameters. Fuel types were represented by an unadditised base fuel meeting EEC year 2000 specifications and the same base fuel plus synthetic deposit control additive packages. Crankcase oils were represented by two types (1) a 5W-30 API SJ/ILSAC GF-2 type engine oil and (2) a 10W-40 API SH/CF ACEA A3/ B3-96 engine oil. The program showed that specific selection of oil additive chemistry may reduce formation of intake valve deposits in GDI cars.. In general, G-DI engines produced more soot and more pentane insolubles and were found to be more prone to what appears to be soot induced wear than PFI engines.