After having to replace the second OEM battery in my XLR within two and a half years of ownership, I choose the Sears' Diehard INT GRP 47. This is a direct replacement battery with the correct post placement and it is rated at 600 CCA. (Cold Cranking Amps)
It's less expensive than the OEM, Optima and the Odessy and if you have a problem, Sears stores are all around the country and they don't charge you an arm and a leg to check out your electrical system! Also, their techs know that they have to connect a support battery/power supply to your system before disconnecting and replacing your old battery! Which means you do not have to re-index your windows or re-program any of your personal settings!
MY first OEM battery had to be replaced on 06/09/08 with only 19,520 miles on the car, at Matrin Cadillac in Bowling Green, KY. This battery had not been maintained with a Battery Tender because I didn't learn about them until the XLR gathering we had that month/year.
The second one had to be replaced on 07/28/10 at 37,612 miles and it was maintained with the Battery Tender from the time I returned home in June of 08 thru July of 2010.
This was replaced with the Sears' Diehard.
Since then, I have left my car sit in the garage for up to six weeks at a time without the Battery Tender on it and have not had any problems with the car battery itself.
Of course both my FOB's batteries had to be replaced while I was visiting relatives in Florida in Oct./Nov. of this year.
To sum this up, IMHO, the OEM batteries used in our XLRs are only good for about two years, with or without the use of a Battery Tender...