Can I add mineral oil to minwax gel to make it less thick?

The directions on the can will tell you if you can thin the gel stain and what you should use. I would look there first. You could try a bit of regular stain in the same color to use as a thinner but not too much or the gel will be runny and not cling as it should.

Minwax does not recommend thinning the gel stain. But I've read on a few forums where others thinned with Mineral Spirits. I woudn't add mineral oil.

Hi. Give it run! Since it's too thick anyway, make a formula of say 2 tablespoons gel to 1 teaspoon Mineral Spirits. Test it on the underside of what you're working on. If it works, hurray! Increase your formula accordingly. If not, you won't be able to see it on the underside.