Here's some info on an oil additive I use that is available here in western Mass (and probably elsewhere). It is called Opti and if you mix it according to the instructions on the container (basically a one or multiple gallon tube) it works for whatever pre-mix ratio your equipment calls for - 50:1, 32:1, etc. It works well for all ratios on the one, single pre-mix. The local small equipment shop I use sells the stuff and I've used it for years with no problems or breakdowns. Might make gas storage simpler at the museum. By the way, I can mix a batch in the winter and it's still good in the summer and vice-versa.