A powertrain control module determines engine oil properties based on friction in an engine. The powertrain control module includes an engine combustion module that models engine torque based on engine speed and engine pumping losses due to intake and exhaust gasses. A transmission module models transmission torque applied to the engine based on transmission speed and gear. An accessory loads module models torque applied to the engine by accessory loads including at least one of a power steering pump, an air conditioning compressor, a water pump, and an alternator. An oil viscosity module determines engine oil viscosity based on a friction torque and predetermined engine data that coincides with reference oils. The friction torque is based on the engine torque, transmission torque, and accessory loads torque.