Abstract The commercial centrifugal oil pump of type 65Y60 has been chosen as a test object, and the affinity law accounting impeller trimmed and effect of the number of blades have experimentally investigated. The results show that the values of the flow rate, head, shaft power and efficiency trim exponents differ from existing trim exponents for normal centrifugal water pumps. The flow rate, head, shaft power and efficiency trim exponents highly depend upon the liquid viscosity and impeller diameter trimmed at best efficiency point. The thicker the viscosity is and the less the diameter is trimmed, the larger the exponents show departure from existing analytical trim exponents for normal centrifugal water pumps. When the diameter is trimmed larger, the test exponents demonstrate a trend closing to the analytical exponents. The effect of the number of blades on the head exponent is the largest, on flow rate exponent is lager and on power, efficiency exponents are minimal.