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I have never had it drain down on me when left sitting. I have left it at the airport for a week at a time. This is the only real issue that I have had.
+1 on the red top part number.
That is when I discovered my problem. Leaving it at the airport.
Other possibility. My cousins 2007 GT had battery cables come loose and cause an inconsistent connection.
If you haven't already, check the ground cable and check the cable where it meets the starter.
As far as the Optima Battery goes, I have no idea on the part number but I can run out later and get it. I went to advance auto parts and said "2006 Mustang GT battery" and thats the one they gave me. It was just shy of $200 bucks though.
OLUCH! I think I paid closer to $155-160 for mine, but that was over a year ago. Love the red top. Have one in all of my cars and never had any problems. Wish they weren't so expensive.
I had the same issue with mine and then one day my car would not start i was pissed so i just went to autozone and got a new one, ever since then i have not head a problem yet (knock on wood).