Here looking at what grease to put on bits like this of an old bike to stop them seizing....but in my experience with other stuck rusted bolts ...cars, lawn mower blades, water taps).
WD40 (or Plus Gas is better if you have any), let it soak for a while and then using a well fitting spanner, attach the spanner and tap the spanner with a hammer in the right direction ...and if that doesn't get it started give it a couple of taps in opposite direction and then again in right direction ..and then try by hand ...
Spray again and leave for a bit longer...and repeat process
I do mean TAPS with hammer not belts as that risks shearing bolts and damaging threads ....and if that fails use an extension tube - and the hammer taps...
Seems the sudden force works better than a build up ...