I own a 2004 MXZ and 2005 GSX, both with 600 SDI engines. When I first picked up the '04 it was using about double the oil of the '05 so I slowly adjusted the cable until they were using roughly the same. It's definitely more than 18mm though... I think every sled is a little different. I use the semi-synthetic blend XPS oil. No major mods.
Just completed a ride last weekend and checked oil & fuel consumption (forgive my Canadian units):
2004 MXZ: (I was riding - a little throttle happy)
270km, 44L fuel, 1.9L oil --> approx. 14.5 mpg, 23:1 ratio
2005 GSX: (wife was riding - much more cautious)
270km, 37L fuel, 1.7L oil --> approx. 17 mpg, 22:1 ratio
They both still seem fairly rich to me, but better rich than lean. It would be nice to be closer to 30:1 but I'd rather not push it. Plugs look great.
Joe