I have heard of using ATF, but have not tried it. I have used Lestoil, an old fashioned cleaning soap found in old time grocery stores. You have to test and keep an eye on it. Might take a few days. BUT I have ruined pieces too, they blow up and dissolve. Be careful and test. I have a set of '5's floor mats that were brittle and would have broken if bent. With nothing to lose I soaked them for days in the sun, kept coating them. That was about 20 years ago, they are still soft and flexible. Is killer for tires too. The black runs off and cleans like the rubber was new.