anti-freeze
If you drain the lines and then blow them out good you can get by with 2 gallons. That is what I use, but I really spend some time to pressurized the lines with air and then open the valve at each sink and toilet, shower and drain points. If you winterized several times a year it would be easier to just use 3 or 4 gallons, as someone else said, it doesn't cost that much. If you don't feel real comfortable, throw in an extra gallon. Also the newer unit may not have low point drain lines, so getting all the water out is more difficult and they may need more anti-freeze.