Bohning Fletch Lac is just a lacquer, any lacquer thinner will work. I use the cheap brand in a metal can from Lowes. Yes, it does need to be thinned to give you a good job. I like it about the same consistency as milk. If you don't thin it, all of the bubbles will never rise to the surface. Sometimes, depending on the color, this may require dipping 2 or 3 times but it is worth it to have that nice, smooth finish.
I always fill my dip tubes, cap it and then let it set for at least 30 minutes. This lets all of the bubbles rise to the surface. It helps to tap on the side of the tube a little bit as well. Then I dip an old junk shaft first to pull any remaining bubbles out before I start dipping the good ones.