The 3K miles interval was made up by the oil companies here in the states, after all, it does make good business sense. Where as in Europe, the same RX300 the oil change interval is at 10K. Having said this, I do think however, that running 10K on dino oil is a bit of a stretch, I probably would not exceed 5K miles with dino oil.
I have been using synthetic oil ever since the first oil change, it is a little more expensive, but I look at it as fairly cheap insurance for our engine. Syn-oil has a much higher flash point temp., and coking is not as prevalence in our hot running engine, it also has a built in cleaning property.
I prefer to use only synthetic oil, running Valvoline Synpower 5w-40 during the hot summer months, and Mobil 1 5w-30 for the cooler months. If I am to use dino oil in my car, Castrol GT is the only oil that I would consider.