The invention relates to a method of operating a hybrid internal combustion engine, which method comprises: fuelling said hybrid internal combustion engine with a combustible fuel; lubricating said engine with an engine oil composition; and maintaining an oil film thickness of the engine oil composition in the big end conrod bearing shells of the engine of at least 0.4 μm; in which the temperature of the engine oil composition is maintained below 123° C. and the engine oil composition has: a high temperature, high shear viscosity of less than 2.6 cP (2.6 millipascal second) at 150° C. and at a shear rate of 106 s−1; a Noack volatility of at most 13 weight %; and comprises: (A) a base oil which is at least one synthetic basestock, the base oil having a kinematic viscosity of at least 2.0 cSt (2.0 mm2/s) at 100 ° C.; and (B) an additive package which comprises at least one dispersant, at least one detergent, and at least one phosphorus-containing, anti-wear additive in an amount to provide a total concentration of phosphorus-containing, anti-wear additives in the engine oil composition corresponding to 0.01 to 0.2 weight % phosphorus in the engine oil composition.