The ratio is 50;1 ie. 100ml oil to 5l of petrol.
NB: This ratio is for Stihl 2 - stroke oil only which is the recommended oil. If you use 2 - stroke oil from a hardware shop / garage, the ratio is doubled ie. 200ml oil to 5l petrol to make up for the poor quality.
After approximately 100 hours of work or when you feel it is not performing like it should.
Drain out all the petrol and start the machine until all the petrol is drained out of the carburetor.