Mobil 1 MX4T is the American trade name for what is normally called "Mobil 1 Racing 4T" in Australia.
You can pick it up at a few places, but only bike shops generally. I get mine from BMS in Beaconsfield (Alexandria) basically for $60 for 4L. Nice discount.
I've only ever seen it in 15W-50 here in Aus as well, although I do believe they do a 10W-40 as well.