I usually wait until my second oil change (about 4-5000 miles) before going to synthetic. As for the time interval, I tend to change at about 4-5000 miles for N/A unmodded cars. With the new oils today I'm sure they don't "break down" like the old ones. Plus, I always use the oem filter or Purolator (oem on the older Subies). Never Fram. I tend to stick with 10w-30 vs. 5w-30. My previous Subie (WRX) would use the 5w oil about 1/2 to 1 quart between changes. Not so much the 10w oil. All this, of course, is a matter of personal taste and experience.