There are many kinds of reducer and I think 1Shot has two if not three from which to choose. Thinner, turps, mineral spirit kerosene and other thinners work, but some might cloud or dull the finished product more than others and adding too much could cause early fading or peeling, not to mention stroke marks.
I always used Penetrol for my lettering and striping. That's close to 40 years now. Just like the other products, you only want to change the viscosity of your paint so it flows better. With Penetrol, you can't do any harm to the paint. Well, you can, but you have to want to louse it up on purpose. However, only add it as you palette your brush. Never add any of these additives directly to your paint or you'll ruin it for later use. Just dip your brush in the paint, dip in a tin of Penetrol, palette and lay it down. You also don't have to dip in your Penetrol or whatever your choice of reducer is each and every time. It will slowly build up on your palette. Every so often, you'll wanna pull your brush dry and get all the paint out of the heel of your brush and you can add a dip or two of thinner at that time.