Have you tried answering Who's there? Sorry i couldnt resist!:-)

 

You can certainly go 7K with the synthetic but, of course, the choice is always yours. Personally, I run 7K between changes; I don't drive dusty roads, usually don't rocket around in the car too much & mostly just drive back & forth to work plus the synthetics don't break down nearly as fast as the 'natural' oils do.
But, I'm sure there are a whole bunch of other threads you can poke around through that are probably a lot more detailed.