Oil brush stiffness is in ArtRage 4 and 5, but not in ArtRage Lite or older versions of ArtRage.

You can check which version of ArtRage you're using by going to Help -> About ArtRage assuming you are on Windows or macOS.

If you have any questions, feel free to contact us via our support page at http://www.artrage.com/get-support/ , we should be able to help.