Wynn’s Engine Flush Professional Formula It is very important to firstly clean an engine prior to adding fresh engine oil as up to 20% of the contaminated oil is typically left behind during a routine service. This “left over's” will contaminate the fresh oil and over time produce sludge & varnish deposits in the engine prohibiting the vehicle to perform at peak performance. Modern engines run much hotter & have smaller cooling system capacities with much less tolerance for clearances as to produce better fuel consumption.