Did you know there are other prescription diets besides Science Diet? Vets always seem to carry SD, but Waltham, Eukanuba and Purina all make prescription diets, too.
Waltham makes a couple of kidney diets. They are naturally preserved, too. NO BHT or ethoxyquin. My sister's dog wouldn't touch the K/D, but loves Waltham. If your vet doesn't carry it or can't get, Petsmart/Banfield carries it, I believe.
http://www.prescriptiondiets.com/rc.html