You can see the original script here: https://www.scribd.com/document/253321759/Mad-Max-2-Script

To understand who he was you must go back to the last days of the old world…

when, for reasons long forgotten, two mighty warrior nations went to war…

and touched off a blaze which engulfed them all.

for without fuel they were nothing. They had built a house of straw…

People stopped in the streets and listened: for the first time they heard the sound of silence.

Their world crumbled…

That script matches the production script you can see here: https://propstore.com/product/mad-max-2-the-road-warrior/262-hand-annotated-production-script/

So, it seems like there was no more oil (the imagery in the script seems to suggest the oil fields being burnt). It is a world-wide apocalypse, although not necessarily of the nuclear kind - it depends how literal you think “touched off a blaze which engulfed them all” is.