Berryman B-12 Chemtool Fuel System Cleaner (part #0116) is compatible with all materials of constructions at the recommended concentration of 1 fl oz/gal. However, it has been successfully used at much higher concentrations, too. In fact, we have heard from customers who have run out of gas and subsequently run their vehicles on pure B-12 in order to get to the gas station! The 5 fl oz/gal that you added to the lawn tractor is high, but it will likely not be problematic. Still, if you were able to get it started and then run it for a while, that would give the product time to clean any deposits while reducing the overall level in the gas tank. Then you could top it off with fresh gas and dilute the B-12 to recommended levels.

Old post, but since then there have been YouTubers that used 100% B12 in a lawn mower and it still kept on running.    I have used only one can in a BMW (which is 3oz short of use for the 18gal tank) but it still significantly upped the MPGs on it ! It hit 31 mpg (: But if this is working similarly to seafoam. Than using 2oz per gallon should work fine if not better. Even so from experience Berryman's gave me better results than seafoam did anyway. And for a better price too. 

I also added a full can to a 30 year old craftsman tractor that only has about a 2gal tank, and it ran without problem and even better afterwards.