Try Wal-Mart or Wal-Mart Supercenter. They seem to carry it for a short while, then clearance it out.... a few months later they'll carry it again for a few weeks, and then clearance it out again.

Some stores carry it, and others don't.

My advice is to stock up when they carry it- especially if you find it on clearance. Normal price is $5.97... sometimes when they put it on clearance they'll mark it down to $3.00/qt. Right now it's on clearance for $5.00 at the Walmart near me.

It's a gray bottle with a yellow box where it says "0W-20".

As to the original question... (5,000 miles or 6 months) I say do whatever comes first. For me, I drive a lot.... so I change it every 4500 miles, which is about 3 months for me.