The recommended fuel mix for this chainsaw is 50:1 which is written on the user’s manual that you get on purchase. There is a bit of confusion regarding the fuel mix ratio for this chainsaw because the earlier models used the ratio of 40:1. So make sure to check the model you are using. The PR4218 model reviewed in this guide uses a 50:1 fuel mix ratio. Also, make sure to use good quality gas which is high octane unleaded gasoline.