Mushroom plug looks slippery. I would think it might be glue. But no, it's just butter. The oil is used to force the cork through the "funnel-like" tool. It was pretty easy to use, but the cork leaked. So I just plugged in and started fresh. This time I used patch glue to ensure the rubber plug and tire were vulcanized together and the hole sealed. But the glue creates a lot of friction between the plug and the funnel tool. I had to try twice and use a lot of glue. It was this procedure that closed the hole. So far a week has passed, there is no air leak.