Miata fuel tank cap
 

1999 Miata fuel tank cap almost impossible to unscrew. If leave loose, Engine warning light comes on. Tried gen purpose lube, silicon grease--works right after applying, but by the next fuel stop it's stuck. Any ideas?

Personally, rather than screw around with it all, coming up
with CELs and just the plain old hassle of it, I would buy
a new cap ($16.95 @ Finish Line/Rosenthal Mazda) and be done with
it. Just be sure to clean around the metal filler neck before you
switch out the caps, and don't get any crud down the neck while
doing so.
Best of luck with that.