In regards to the auto-loaders and selection of ammunition, there's a few ways that selection of different shell types can be achieved.
1. The auto-loader ammo store has reserved slots for the different shell types and the gunner literally hits the button for the desired shell and that shell is loaded.
2. The auto-loader has no reserved slots and the crew have to decide what shell type to load the ammo store with before a fight.
3. The ammo store is filled with the most commonly used shell type but the crew can replace/refill slots on the ammo store so that if, for example, HE is desired but there's only AP loaded into the ammo store, the crew remove the AP shell next in line to be loaded with the desired HE shell.
Now having said that, I know this from some theory on the subject but I couldn't tell you specifically what tank (or even what country) uses what method. From what I recall, the T72 has option 1 and the gunner selects what shell he wants to use and the ammunition carousel rotates to place that shell in line for loading.
How much less time this takes compared to a human loaded is debatable but what having an auto-loader does achieve is a reduction in size and weight of the tank and with a lower superstructure the Soviet belief was that their tanks would be harder to see and somewhat harder to hit.
P.S. This page has some internal diagrams of the T72, I found it when I was checking my info for the above and if you go to pictures 6 & 7 on the page, there is a description of the auto-loader and ammunition carousel that explains things better than I did.
http://panzerfaust.ca/AFV interiors/t72a.html