I bought some Sears Craftsman 40:1 2-Cycle Fuel Mix to use in my Craftman 2-cycle trimmer. Anyone know if I can add MMO with this pre-mixed 2-cycle fuel in my trimmer and if so what is the ratio?

I would just add 1/2 ounce of MMO to your 1 gallon mix and run it that way. I have a more complicated formula that I run in my 2 strokes for testing purposes but you need a precise way of measuring and most people don’t want to bother. Marvel recommends replacing 25% of your 2 cycle oil with MMO but, once again, if you don’t have a precise way to measure, it’s a problem.

An even easier way to do it is to buy 50:1 ratio bottles and add 1 ounce of MMO per gallon of mix. This would make your ratio 35:1. It won’t harm a thing and is much easier to measure.

Normally, we would recommend that you replace 25 % of the the 2-cycle oil with Marvel Mystery Oil.  For example, if normally you are using 8-ounces of 2-cycle oil, use only 6-ounces and 2-ounces of Marvel Mystery Oil.  Since you have already mixed the 2-cycle oil with the gasoline, you can add about 1-ounce of Marvel Mystery Oil.