I have not used the lighter fluid hand warmers since the 60s, I assume that is the type the Zippo is, back then it was the Johnny warmers everyone used. They leaked lighter fluid and they would get too hot and you had to take them out of your pocket. My dad used a warmer that looked like a wide hard glasses case, it burned charcoal sticks or at least that is what they looked like. That worked a little better and didn't leak or stink like lighter fluid. All I use now are the hot hands warmers that you just shake to get the chemical action in the iron filings to start. I pit one in each coat pocket, they last 6 or 7 hours. I always carry a few extra, they take up no room in my pack just in, case.