You could try cleaning the bottle neck before blaming the polish. If there’s dried lacquer around the rim, the cap never seals fully and the bottle keeps thickening no matter what thinner you use. I use a tiny bit of acetone on a lint-free wipe for the outside threads only, then recap tightly.
Edit: if the brush is permanently crusty or the polish has hard chunks that won’t dissolve, thinner probably won’t save that one.
OPI thinner works across my whole collection without any issues. Acetone breaks down polish over time and that made things messy for a while. It might work in a pinch, but I wouldn’t rely on it regularly. Worth getting actual thinner if you want your polishes to last.
Check the ingredient list and look for only butyl acetate and ethyl acetate. Heptane will wreck your glitters over time, which is why I only use KBShimmer for anything glittery. You have to order direct from their site though, since Amazon doesn’t carry the thinner.
For basic creams and cremes, the Morgan Taylor on Amazon does the job for under ten bucks.
OPI’s thinner works well once you get the amount right, but it needs a little bit of trial and error for each different polish to figure out how much to use.
Try Orly’s dropper bottle. The built-in dropper makes controlling how much you add easier than the other bottles I have. Worth a look.
OPI’s thinner has been solid for me. Mine’s worked on polishes that are five years old, no issues so far. Just haven’t tested it on HT formulas.
The dropper cap is what sold me on KBShimmer’s thinner. They include both a regular cap for storage and a dropper cap for actual use. It’s convenient if you’re already grabbing brushes or topcoat from them anyway.