Hi all, first post after many months of lurking...
I was a Ford Technician working in Perth Western Australia way back when the AU series was launched, and I've owned a lots of Falcons over the years. Rest assured, they aren't fussy about oil! 15w50 won't worry the old girl in the slightest.
I've run 20w50 in my current AU, and even on frosty Tassie mornings, she fired up straight away, with no knocks or rattles.
You'll be fine!