Had a slow drip leaving oil on the garage floor. On my last change I decided to use a copper gasket that was the right size although I had some hollow steel Subaru washers. I'd bought it years ago for use in a Honda manual transmission fill bolt (same 18 mm internal as Subaru bolts).
After removing the underspoiler, it was pretty obvious it was leaking from the plug/gasket. So with a quick motion, I removed the plug and covered up the hole with my gloved thumb. Then with my free hand I removed the copper washer and placed the new steel one on and returned it and tightened it. I couldn't have lost more than 3 oz of oil - probably less. It was Mobil 1 5W-30, and I didn't particularly want to pour the drained oil back, in case there were large metal shavings in the pan.
I looked at the copper washer, and the crush marks were all around the circumference except maybe for a 5 mm section that didn't seal (no mark). I was rather surprised, as I had really cranked it tight. Anyone ever have a washer not seal even when cranked nice and tight?