I was going to check the fluid level today and either my allen wrench is rounded or the plug itself is. I'll try a new one next week, but just in case it's the plug, what is the best way to get it out?
Use a proper sized shorty extractor (size depends on how hogged out the original hole is...get an assortment) while the unit is hot/warm...break it loose and then you're in the clear.
Note that the cavity can build and hold pressure, even when cold, so take care when removing the plug or you'll take a bath - and that stuff stinks.
Normal size is 3/16 inch allen. Be careful putting it back in. First, don't drop it! Second, don't tighten much at all - just snug. It always works tighter and the steel in aluminum can be a pain to open next time. I always just barely snug it up.