sharpening stones

I have a couple of oil stones that I always use for sharpening my chisels and handplanes and a slow speed Delta grinder, but I came across a tri-stone (the 3 sided block with the course, medium and arkansas stone) for an excellent price so I grabbed it. My question is that it states that I can use oil or water for the lubrication, what is the preferance in your shops? Do the stones last longer with one verses the other? Just wanted to get some tried and true advice before I use mine.
Thanks again,
Mnwoodworker