hi im not sure if this is a really good one to try, becasue of any side effects on the carpet, but whenever i used to get tart on my feet as a boy, my mother would clean it off with butter/margerine as i remember it came right off
I got one of those tiny Stain Devils especially for removing oil and tar, it worked a treat, there was no sign of the tar after use, but not sure if you can still buy them, they used to be sold in chemist stores mostly. good luck