How to thin UV sensitive resin I have some older resin that got a bit thick. I was thinking I could use some paint thinner to thin it a bit and restore it. My thoughts are the paint thinner is made of terpene oils so should be compatible with the resin.

To test, I went ahead and mixed a half capful of paint thinner with about a cup of resin. I mixed it thoroughly and it seemed to mix well and restored proper viscosity. I then filtered it through a paper coffee filter twice. The results were very nice and smooth resin. I did a UV hardening test with a few drops on a strip and into the UV lamp. It still hardens. 

Any thoughts, suggestions or OMGs! before I try this in my printer? It's a Pegasus Touch SLA.
You've covered a couple of concerns "outside the box," but the real tests will come when you pour some of the resin into the vat. 

If you don't get a reaction with the release layer, score one for your side. The release layer is a silicone kind of compound, which typically is un-reactive to most substances, but that's not to say the thinner qualifies in that respect.

To be cautious, you could dab a bit on the corner of the vat, especially out of the usual scan area or off the build plate contact surface. If it sits without clouding the release layer, the next test would be a layer test.

You could even remove the build plate, pour the modified resin in the vat and run a model comprised of only one layer, perhaps two. You'd be able to carefully pull the cured resin off the release layer and make a preliminary determination.

If all is well by this point, it's time to create a model, yes?

I hope you'll report back on your results. My Pegasus is still in the box from Kickstarter days. I'm sure my resin is well beyond the "sell-by" date!