The 4AFE is different from the 1ZZ and 1ZR engines... this is the deal:
As per my research, a combination of piston rings designed for low sulfur fuel, a lack of piston-cooling and the nature of the Z engines to run hot in traffic causes the sludging. Synthetic helps slow down the breakdown and sludge formation, but it's not a cure. Some US owners report oil consumption of over a liter between intervals.
That's the price you pay for fuel economy.
The new ZR has improved rings and presumably better piston cooling, but Toyota is still sticking to the 5k interval.
For all local manufacturers, you get the 5k interval unless you use synthetic oil. If you want to go 10k, it makes a whole lot of sense to go synthetic, anyway... unless you want to collect engine-sized paperweights.