I changed my oil from regular to synthetic at 15k miles at Jiffy Lube. Later I noticed the sticker they placed for my next oil change at 18k miles. I thought I should be able to change synthtic at 4500-5000 miles. What's going on? Do I have to change it at 18 k or 20k?

 

You should be fine with the higher interval. Keep in mind that Jiffy Lube makes money every time you come through the door...if they can sell you more expensive synthetic oil AND keep you coming back every 3K, that's better yet (for them).

 

I would doubt if they really put the synthetic you paid for into your engine. I would do my own synthetic oil change anyway.