If PB Blaster is 10X better than WD-40 or plain lubricating oil, KROIL is 20X better than PB Blaster. KROIL = "creeping oil". Cano labs says it will penetrate into 1 millionth of an inch. I don't know it it will do that or not but some of the things I have used it on have deeply impressed me.
The last tough job I used it on was a stripped/broken off flush, tractor gas tank fitting. I sprayed it, tapped in an easy out, and it walked right out. Prior to that it was a sewer pipe cleanout fitting that had not been removed for 25+ years. I sprayed it twice with an hour between each, and it unthreaded without issue but it was tight the whole way.
Good thing you guys got that fill plug out ok as stripped alum. is a bear to helicoil. Especially something that you cannot drill out without shavings getting inside.