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As part of General Motors effort to improve fuel economy, the effects of engine and power train lubricant viscosities were investigated in passenger car tests using either high- or low- viscosity lubricants in the engine, automatic transmission, and rear axle. Fuel economy was determined in both constant speed and various driving cycle tests with the car fully warmed-up. In addition, fuel economy was determined in cold-start driving cycle tests. Using low-viscosity lubricants instead of high-viscosity lubricants improved warmed-up fuel economy by as much as 5%, depending upon the differences in lubricant viscosity and type of driving. Cold-start fuel economy with low-viscosity lubricants was 5% greater than that with high-viscosity lubricants. With such improvements, it is concluded that significant customer fuel economy gains can be obtained by using the lowest viscosity engine and power train lubricants recommended for service.

A 55-taxicab fleet test was conducted for 160 000 km to determine the effects of low-viscosity and friction-modified engine oils and rear axle lubricants on fuel economy and durability. In GM City-Suburban and 80 km/h constant-speed tests conducted at the Indianapolis Motor Speedway, fuel economy with low-viscosity engine oil and rear axle lubricant combinations was about 1 to 2 percent greater than that with SAE 10W-40 engine oil and SAE 80W-90 rear axle lubricant. These improvements were obtained after 40 000 and 80 000 km of taxicab service. Fuel economy with a graphite-containing SAE 10W-40 engine oil was about 2.5 percent better in both tests. No differences in either engine or rear axle durability were observed among the lubricants. However, oil economy with low-viscosity engine oils was about 20 to 30 percent less than that with the SAE 10W-40 oil.