Time for me to rant! What does it matter?
This is not to impugn or insult anyone when it comes to a question like this.
But honestly, if you want to use a vintage machine and know full well that it uses more water than the dish wetters for sale now, then why is it so important to know how much water it uses?
If water costs are a real premium where you live, your hot water heater has inadequate capacity or recovery and precludes using a maximum of 15 gallons of water to get your dishes clean and not have to rewash them, then, Yes, it is a true concern.
The features that truly stand out for me between the 2 machines is the fact that the 18 has 3 after rinses and will heat the water to approx 180 degrees when using the Sani Cycle (2 rinses on the short wash cycle) and the 20 provides only 2 after rinse and sanitizes at approx 165.
They share the same pumps and wash arms with the 20 lacking the constant rinse up top. The 20 does at least heat the water to 140 but as many have mentioned on this forum, once heated, it sprays on to the cold dishes and tank and immediately drops in temperature. Why Hobart never had a heat delay while washing is something we may never know and it was a good change that WP made when it came to the 23 series.
Since I am on a rant already,
Another ridiculous concept was the energy saving dry option.which was probably mandated by the government or added to every machine due to competition.
I appreciate saving energy as much as the next person, but if you run the numbers on the savings by using no heat, you are talking probably less that $.05 per load to have the heater provide heated air to assist drying the dishes. This would actually be less since the elements usually don't use more than 700-800 watts but the math is easier with round numbers so you can see what a kilowatt would cost and then divide that cost by 3 in this case. I never use the option and prefer as much water to be dried off my dishes and glasses as I can get.
Sorry to rant but I just had this argument with a co-worker and have not cooled off yet!