I use a lot of epoxies manufactured by Abatron, and they recommend thinning with xylol (aka xylene), available at good paint stores. I use lacquer thinner for cleanup, or for thinning when I am out of xylol. Xylol is a component of lacquer thinner.
Re how far you can thin epoxy before you degrade it, that depends on what you want the epoxy to accomplish. I often need to reconstitute punky architectural wood (as a result of termites, dry rot, etc.) and I have found that, for that purpose, thinner is definitely better. Thinning ratio depends on the viscosity of the original epoxy, but for my purposes I sometimes go with 2pts xylol to 1pt epoxy for the first few applications. I apply to the end grain whenever possible, and just keep applying as long as the wood will soak it up. Don't get the epoxy mix on any surface that will receive a transparent finish.
