I was taught every 5000 miles. Back when oil technology was in the 70s. Newer formulations can probably go a LOT longer, like 7500 or 10000 miles or your 15000. What we aren't addressing is what kind of environment is the engine running with these rules of thumb. Your doing highway miles, but is it really dusty, dirty, and with large temperature swings and high revs, or lots of traffic jam/kiddi carpool line idling? You seem really kind to the accelerator (rare but still brilliant). So unless you are in an extreme environment, there's a good chance you can easily get 10k+ between oil changes.

Other factors come into play, of course -- what's the mileage/time on the engine (wear plays into blowby), how clean the air filter is, perhaps even the quality of the fuel. Without the ability to test for viscosity breakdown, metals or soot in the oil, we're just kinda guessing.

 

Thanks. I live in Texas, so the answer to driving conditions is, HOT. I live far from the city and it is about a 70 mile round trip drive just to go to Walmart. I also have routinely drive to mulptiple parts of the state on a weekly basis. For example, I have drive across Texas back and forth twice in the last two weeks. I also keep clean air filters, as that increases fuel economy, which is big for me. I own a big 3/4 ton 4X4 truck, which have not driven in four months because of gas prices. My 2013 Sienna LE has 175k miles. My last Sienna, a 2004 model, had 300k miles and ran like brand new.It did not burn one drop of oil and got around 25 MPG (I got 28 mpg out of it one time). I would still be driving it right now, but not one, but TWO deer decided to kamikaze themselves into it.

Anyway, from what you said, I feel I should be safe going 7500 miles between oil changes if I stick with this full synthetic stuff. The way I drive, I probably could safely go 10k, but I would worry too much and that stress is not worth the savings.