Re: At 200,000 miles, how long can I reasonably keep going?
There are a number of 1NZ-FE engines from the xB, xA, Yaris, and Echo that have gone past 300,000 miles. High mileage oil isn't necessary unless you have consumption or leaking - it can help condition brittle seals and the like. If high mileage oil helps you worry less about your ride, it won't hurt anything. Really, our engines aren't picky about oil - as long as the conventional oil you're using is API or ILSAC certified and changed regularly, you'll be in good shape. Even the store brand oils from the parts stores and big-box retailers are fine - they're all bottled by major players, and have the appropriate certifications. Stay away from the true no-name oils - the state of New Jersey just banned a bunch of them after testing revealed they were contaminated with all kinds of stuff. It looks like many of them took recycled oil and re-bottled it without refining it again.
Make sure you keep up on the other fluids, especially coolant and ATF (if yours is an automatic). Do those things and the engine will outlast the rest of the car.
Suit the action to the word, the word to the action.