Recommended Pan Plug Seal

[DT1]

I've had an oil leak for some time now, so I changed my oil a few days ago and found that the pan plug had somehow worked it's way loose. Not so loose to fall out but loose enough to leak. The only reason I can think this would occur would be a damaged or incorrect sealing washer.
The last time I changed the oil I used an aluminum crush washer, so I figured I'd try a different material. This time I installed a nylon washer and it didn't leak a drop for three days.
I backed the truck out today and I noticed oil on the floor. When I checked the pan plug, it was loose again.
Now I'm not sure what material sealing washer I should use.

In both cases the plug was tightened properly.

Any recommendations will be appreciated.