bad, which is why 10k is the general rule of thumb for a quality synthetic.

If you are sampling on a clean motor you could theoretically go for a very long time, but very few people do.

But there is so much "margin of safety" built in to the 10k service change interval I really don't get the idea of changing every 5k with pure synthetic. If one is doing 5k oil change intervals (nothing wrong with that, especially in a severe or dirty duty cycle), buy a cheaper oil.

The Synthetic oils don't lubricate any better....they just last longer before they break down. I guess technically they hold up to heat better (that's why they last longer), but I'm not sure there is much benefit over a quality non-synthetic.