A lubricating system and method of operating the system for an internal combustion engine wherein the amount of lubricant supplied to the engine from a positive displacement engine driven pump is varied by selectively delivering lubricant from the pump to the engine or bypassing the lubricant back to the return of the pump. The amount of lubricant supplied is varied by changing both the duty ratio of the lubricant supply and the duration of engine supply in response to a map of engine conditions. Various control routines are disclosed and in each the lubricant supplied to the engine is monitored so that once a finite amount of lubricant is supplied, lubricant is not supplied again to the engine until that amount previously supplied has been consumed.