I have tried this a couple of times. It is hard to get just one gallon. And in all honesty you really do need more than one gallon to quench appropriately. Now I tried testing this with a gallon of Canola Oil that I was using and the one gallon of oil got really hot after just one knife ~200F. Now that is hard to say there is something wrong with that, if you are going to quench a single knife. If you are going to quench 2 knives you either have to wait or use another individual gallon of oil to make sure your knife is quenched right. But think about it. With more medium, there is a better heat transfer, a quicker quench which should give you better results depending on the type of steel you are using. I personally want to make the best knife that I can at that time. And I believe that quenching in a gallon of any oil is not giving the best results because its not transferring heat fast enough because there not enough medium to keep transferring the heat from steel to oil.
Most makers will tell you that you need more than one gallon. If you are going to use Parks 50 you want it to cool down your steel really fast. So having enough to quench the steel is important.