Gearbox filler plug seized

Given that its oil inside (not water) then it seems more likely to be tight than seized. (corroded). A socket or ring spanner only as others have said. Some diesel or kero on the external might help. WD40 isn't much good for this type of thing, its a water displacement (WD), but ordinary diesel does quite well.
 
Given that its oil inside (not water) then it seems more likely to be tight than seized. (corroded). A socket or ring spanner only as others have said. Some diesel or kero on the external might help. WD40 isn't much good for this type of thing, its a water displacement (WD), but ordinary diesel does quite well.

so why not sort it on assembly, torque to sensible limits & lube the thread
 
No one's mentioned the application of heat. Would that not be a good precursor to applying force to a suitable spanner? Especially if applied to the alloy casing around the filler nut, rather than the nut itself?...

Yes, done with care. And a bit of gentle tapping to the plug (hammer and pin punch) might also help.
 
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