The purpose of this research is to perform comparative analysis of the effect of fuel additive SO-2E on performance parameters of direct-injection Diesel engine, when operating on Diesel fuel and shale oil alternately. Multifunctional fuel additive SO-2E proved itself more efficient when applied in shale oil than at application in Diesel fuel. At the light-load operation the consumption of treated (0.2 vol%) shale oil based upon fuel energy content throughout speed range 1400-2000 min(-1) diminishes from 14.6-12.3 MJ/kWh to 11.6-11.8 MJ/kWh or by 20.5-4.1%. The maximum NO and NO2 emissions for treated shale oil are lower by 22.9-28.6% and by 41.6-13.4%, respectively. Opacity of the exhaust gas and CO emissions for both fuels treated are a bit higher.