I have the same problem, 2 40:1 and 2 50:1. My problem is I do not use enough of either to mix up 2 separate gallons. My Sthil dealer sold me some universal oil that should work with anything to 50:1, but the 40:1 trimmer does not seem to like it as well. Any suggestions? Would the 40:1 work in my Sthil chain caw and Ryobi trimmer ?