It shouldn't be super tight and I don't think you have to mess with it when it's hot from use.
The torx end of the scrench that came with the string driver that comes with the FS 130 Stuka fits that,I think it is T27 . My grease plug is some kind of rubbery plastic. Don't remember whether my sickle bar/hedge cutter that mounts on the power unit attachment has one of those plugs.
neither of the two hex ends on the scrench fit the outside of the plug.
instruction says to check it every 25 hours of running which would probably be about 2 years for a homeowner,only if no grease can be seen put up to 1/5 oz. from the Stihl grease toothpaste tube in but don't "completely fill the gear housing with grease."
might be a good idea to clean the area and the torx hole in the plug before trying to get it off--don't want grass crud to get in there and also torx in general get stuff in the hole ,then people don't get the driver in deep enough and they wallow the hole out.
One main thing to remember about using that bad boy (much noisier than the little homeowner weedeater) is that if you put that heavy sicklebar thing in place of the string head/gearbox,you need to reposition the handle,otherwise you beat your arms to death working against the weight of the thing.
now I am curious about the hex size that fits the outside of the plug but will wait until daylight and maybe try --one of the oddball obsolete 32" inch fractional size sockets?---------------
--------it's a sloppy 1/2" (12 mm. is too small) ,if the torx wallowed out my guess is a pliers wrench would take it off or a 1/2" socket in 6 point-would not trust it not to round with the 12 point wrench or socket