Numerical modeling of friction in elastohydrodynamically lubricated contacts are of high importance in the development of various types of machine elements such as gears, rolling element bearings and cam followers. The friction generated in the machine elements of a system does not only affect efficiency, but also the dynamics and overall function. Accurate simulation of the friction generated in machine components is, among other things dependent on realistic lubricant properties, which may change during ageing of the lubricant. Many modern machines are in operation for several hundred ours before the oil is changed. In this work, two engine oils, one 10W-30 and one 5W-20, have been aged in full heavy-duty diesel engine bench tests for a duration comparable to the amount of ageing these oils are subjected to between oil drains in field conditions. The aged oils were subjected to several analyses showing, among other things, a maximum increase in viscosity, and a reduction in some additives. Fresh and aged oils were tested in a ball-on-disc test rig under elastohydrodynamic conditions where friction was measured. The results showed almost no difference in elastohydrodynamic friction between the fresh and the aged oils.