What quantities of oil to petrol for stihl trimmer?

Just go to a Stihl dealer he has oil in the right quantity for 1 gal or 5 of gas. Competitive price- about 1.50 for enough for 1 gal- then you know it's good oil and the right oil. If you buy cheap oil for say .99 then you saved 50 cent- how far will that go replacing or repairing- not very