Good quality engine oil lubricates moving parts in an engine and protects against harmful deposits building up in the exhaust. Adding Engine oil to fuel in a ratio of 1 part oil to 50 parts fuel is common in Stihl 2-Stroke Engines. To work out the correct ratio mix for 2-Stroke Engines, 1000 divided by the ratio equals the ml and then the litres of petrol times by the ml equals the oil required.