I have re-evaluated my Sanskrit re-creation of the
Amitāyus mantra from the Tibetan, which is on the
Amitābha page. The Tibetan reads (in Unicode)
ཨོཾ༌ཨཱ༌མ༌ར༌ནི༌ཛི༌ཝན༌ཏེ༌ཡེ༌སྭཱ༌ཧཱ།
This is usually transliterated:
om amarani dziwantaye soha which reflects Tibetan pronunciation. The actual syllables are:
oṃ ā ma ra ṇi dzi wan te ye svā hā, from which I have now reconstructed the Sanskrit:
oṃ amaraṇi jīvantaye svāhā
ओं अमरणि जीवन्ताये स्वाहा
More details and Siddhaṃ calligraphy on the
Amitābha page. Ideally I'd find a Sanskrit source for this mantra to clarify the matter - if you know one please let me know.