Since the gas treatment is for the gasoline not the oil/gas mixture I would put the stabil in the gas can first before adding any oil. Now you can use the gas for anything and it will stay good for a while longer with the stabil in the gas.

If you want to mix some oil in the gas for 50:1 then mixed one can of 50:1 oil in six gallons of treated gasoline.

My boat's external gas tank is 6 gallons so this work for me. You may need to use more gas and thus more oil and you will have to measure the amount of oil to use by knowing how many gallons of gas are in your tank.

Or you can make up several six gallons of gas/oil and poor them into your larger tank. <-- it would work but it would be a pain to do.

they make measuring devices that tell you how many onces of il to use for different gas mixtures.

For my Echo Gas trimmer which also used a 50:1 gas/oil mixture I use one can of the echo oil to one gallon of gasoline. I also add the stabil to the gas first then mix the gas with the oil and put in a separate gas can that's marked for Echo 50:1 ratio.