The 18 comes with a ambigious caveat: for all flight conditions or somesuch. The aux tank is all useable; it's V shaped whereas the wing tanks are square corner affairs.
As Larry, I too have burned nearly 20 gallons from a wing tank. This was in straight and level flight. I watch the fuel pressure needle and when it starts getting twitchy I switch. I've yet to run a tank completely dry, as I do in my Luscombe, probably I believe in gravity more than I believe in fuel delivery systems that defy it
In short, what I believe they mean is that up to two gallons may be unavailable in a climb, descent, or in unusual attitudes. Many aircraft have such limitations. I'm sure you've all seen fuel gauges that have a mark for minimum fuel for take-off. This is based on the attitude for climb and the possibility of fuel sloshing to the rear, exposing the intake port. It's also a reminder, apart from technical reasons, to put some damned fuel in the airplane
As we should neither take-off nor land on the aux tank, on those aircraft equiped with an aux tank, I'd count on close to 20 for cruise, and 18 on the tank you'll be using for either taking-off or landing.
Cruisairs are decent gliders, as Larry notes, Cruisemasters....not so much, but this theoretical four gallon loss should be moot IMO, because I always land with more fuel than that. I may be a Massachusetts Liberal but, when it comes to fuel, I'm a Goldwater conservative :wink:
Jonathan