I had the same problem on my 90 when I got it. First I got 2 new plugs, then tried to get the fill plug out. It had been overtightened since a PO neglected to put the copper gasket on. I got a grinder up there and made the flats broader so I could get a wrench on it -- no go. Eventually ground so much away there wasn't enough to get a wrench on. Let it go a couple weeks and noticed it was a little wet with oil. Was then able to turn it out using a cold chisel. My guess is the heat from grinding (actually an air cut-off tool) heated things up to allow the ATF to work through.

I always use a 13mm 6-point socket on these plugs. Using an adjustable wrench would be asking for a repreat to this whole ordeal.