Bear in mind that engine oil deteriorates not only base on usage, but also depends on climate condition, air condition, driving habits, etc.
As in tropical countries, where humidity and temperature is high, do not expect it to deliver the same result. Just like the tyres, don't wait till mileage arrives 40K - 50K then change, but 2 - 3 years whichever comes earlier. I would think that changing of the Amsoil atmost 15K would be the max.