Two Dimes is a good resource for this kind of information.
For high only, AAA is a .722/.278 favorite against 555. For stud/8, the numbers change only slightly to .714/.286.
Absent any other information, if you start with one low card, the chance of making a low are (28/49) * (24/48) * (20/47) * (16/46). I make this to be about 4.2%, or 22.8:1 against. A longshot, but hardly earth-shattering.
When you have a three-card low, the odds are about 2:1 against making an Eight-low by the river. AA3 is about a .645:.355 favorite against A56.