The instructions are for EV olive oil - but many people have had success with lighter oils. I have no input on grapeseed oil, so I can't comment there.
The idea is to consume a large quantity of oil to induce the gallbladder to release a lot of built-up bile to 'push' the 'stones' out. (The Epsom Salts help dilate the bile ducts so that larger 'stones' can pass through.)