@NTrav wrote:
“My drain plug had a combined flat washer w/rubber washer instead of a normal crush washer. I had never seen that used on a drain plug before and didn't take the time to research it so just put it back on because wasn't sure if a crush washer was wrong for the drain plug in a sump cover. “
Well, the combination of a flat metal washer and rubber washer effectively creates a reuse-able “crush” washer. The trick, in replacing the rubber washer over time (when it gets petrified and leaks), is to get a rubber washer that is a push-on tight fit onto the threads of the plug, and also has the same OD as the metal washer and is maybe 1/8” thick. That way, when they’re tightened as a pair, the rubber washer squashes, making the ID smaller to seal against the plug, while also getting pressed against the sump plate. Interestingly, our Subaru Outback has a (very thin) replaceable metal crush washer, while my Nissan Rogue has a rubber O-Ring and no metal washer.