When I pick up my Ranger used it had a flat on the rear. When I got it home I found the small leak in the tread. Sealed it off with slime. Month and 1/2 later I bought a pir of mud grip tires for the back. When we changed the tires over, the slime in that old tire was black goo around the inside of the tire. Some of it was froze like thin ice. Tire still held air OK so the slime was still working but sure looked ugly. I guess what you don't see or know about don't matter as long as it keeps working.