I've read on another thread a while back that the Toyota "sludgemaster" V6 engine had its oil return channels drilled larger to elliminate the pooling responsible for the sludging issue. Presuming that's so, and that Toyota has consistently been noted for basic design goodness, I doubt you have much to worry about, now. Nevertheless, most manufacturers have a two-tier policy regarding oil change intervals - "regular" and "severe", with "severe" specified at 3 month or 3,000 mile intervals. It seems to me that if you're concerned at all, go with the "severe" recommendations with a good conventional* such as Pennzoil, Castrol GTX, Havoline, or Chevron Supreme - the latter two typically very attractively priced with Havoline typically being available at more locations**. If Toyota is like most other manufacturers, they allow the use of synthetics, but don't differentiate longer oil change intervals to maintain the warranty. My SOP with my 2003 Hyundai Sonata is, if Hyundai wants to see "severe" service intervals of 3 mos/3,000 miles over the duration of the 10yr./100,000 mile powertrain warranty without blinking an eye in case I have to submit a warranty claim, why should I spend the extra money on expensive synthetics? For what it's worth, I changed the factory-fill oil and filter at 600 miles, and then again at 3,000 miles. The engine's been on a 3,000 mile regimen ever since. (But then, I live in a temperate climate and enjoy any excuse to crawl under the car...)
*Most current conventional 5W-30 motor oils have significant if not total usage of at least Group II hydro-finished base lube stocks.
**Both Chevron Supreme and Havoline are produced by ChevronTexaco. The product data sheets available online in PDF form are identical, grade for grade, save for a 0.2 higher figure for the Total Base Numbers (faovoring Havoline). Real or typo? You be the judge. But, it doesn't sound very significant.