If you are in Adelaide, 20W-50 got you this far, it will keep you going into the future.
I wouldn't see a need to go synthetic, as given the $40 price difference, you'll need to save a fair amount of petrol, which is unlikely.
I spent a decade living in adelaide as a kid, when 20W-50 GTX and XLD were all that was there. Dad's R-16 made it to 380,000km+ (odo broke), even after we moved to places where it DID freeze.