I never use sealer on transmission pans, but some people do successfully. I do however like to use #M yellow weatherstrip adhesive to glue it to the pan and simply hold it in place. Like cuda65vpt said, if you overtorque the bolts (a LOT of people do) the pan WILL leak. The torque on them is only like 8 foot pounds. Maybe less. If you torque the pan until the pan touches the transmission case, you've over torqued it.