Dcraig - my '87 560SEL has about the same gauge swing as you described. I've often joked with my wife that as long as I'm turning right, I don't need to buy gas .

One thing I like about this fluctuation is that when I'm near the bottom of the tank, I can tell if there is definately still a decent amount of fuel by whether the gauge moves when I turn. If it doesn't, better hit the pumps soon.

On a related note, I've never seen the low fuel warning light go on except on starting the car. Any way to test this other than to run the tank dry?

jlc