To coin a phrase, "oils aint oils". :moon: Most importantly thing is to change it regularly.
Having said that, I have no idea about the difference between Mobil 1 AND Mobil 17. But as above, I think most people tend to stick with old faithful, whatever that means for you.
But since Carbonboy has seen fit to taunt me, the "Toyota Emission control & maintenance guide - 1986" stipulates Grade SE, SF or higher grade. This classification system is obsolete these days.
Essentially use a "good" multigrade oil. 10W-40, 20W-50 or whatever, which is better suited to the Australian climate, unless you live on the top of Mt Kosciuszko.