i have 01 4 runner, needs to remove rear differential oil, the
filler nut is hard to remove, would you suggest using propane flame
to heat the nut, then turn it to open ? will that work

I would be very cautious about heating that plug. You might crack the housing and then you have a real expensive repair on your hands. Try penetrating oil like Blaster and see if that helps.

PB blaster plus a socket and breaker bar.

I don't know what that plug is made of, but doesn't heat generally have a "swelling" effect on metal?

Yes, that was my problem. You would have to heat the housing and I don't like the idea. Never done it though. I would go the Blaster route and see if the penetrating oil allowed you to turn it.

PB blaster plus a socket and breaker bar.

curious to know what is PB blaster, i know if i fit a copper pipe over
the rachet and use hammer to move the nut.

PB Blaster is penetrating oil.