Yeah, you have to tighten the plug as lightly as possible because the aluminum will split the second you pick up the ratchet. You don't even have to put the ratchet near the oil cooler. Just get it close and think "tighten" and the aluminum splits.
IIRC that washer was aluminum so it is hard to anneal (soften) but if it is a cooper one you can heat it with a torch until it glows dark red and dunk it. That'll soften it so that it'll seal. But since you' tightened it as hard as you possibly could the washer probably doesnt matter.
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