I buy my mixing oil by the gallon and refill my 4-ounce bottles as needed. About $12/gallon at Wallyworld. Been running it for prolly 15 years now, with nary a problem. I mix 4 ounces to a gallon in winter, for 32:1, while I often stop on 8 or 9 tenths in hotter weather, so I have a little more oil in it. A friend of mine who used to cut timber often ran his at 20:1 to 24:1, and had his saws screaming.