WD40, Plus-gas, GT85, people get all prissy about one being better than he other or one not being "proper penertrating oil"
The truth is they all work and all have a similar properties as found with the lighter distilates of oil, people just like to think they are being profesional by using something boutique or pricey, they aren't BTW.
Seized stem - give it a soak with a WD40 as you have done, don't listen to people telling you to buy something else.
Wind the bolt out and give it a tap to free the expander wedge, see of the soaking frees the stem when you give it a twist.
If this fails find something to plug the bottom of the steerer and fill the steerer with spirit through the expander wedge bolt hole on top of the stem, let it soak for a few days then try again.