Ron,

Not sure exactly what the problem is but of you don't have a second
wrench, ate you at least letting the blade cut into some wood or
soemthing to keep it from spinning as you try to release the nut.

Having the blade cut into so material as you try to loosen tne arbor
nut ensures you are trying in the right direction.

The other this to keep in mind is that vibration is the least resitable
for for metalic opbejects so some "light" tapping with a hammer on the
end of the wrench can encourage a stuck nut to come loose.

I change the blade 2-3 times a day when I'm in the shop so I've never
had one sieze up but those acme threads can sieze that tightly that a
little elbow grease won't cut them loose.

Finally, you could soak it with some penetrating oil/WD40/rust breaker
for a bit first. Frankly I think oil or not on the arbor is really no
issue. We always left it dry but just had to replace the arbor on a
Unisaw and thje new one came drenched in oil and we've just used it
like that.