The power steering fluid level seems a bit low to me --- I'll have to check that again tomorrow morning on a cold engine.

What can I use to refill?

The manual gives a GM part number for the fluid, but to get some of it here would probably take about two weeks or more. If the level is actually as low as it seemed, I need to refill ASAP.

Can I fill up with Dexron III? What fluids are comparable to the GM fluid the manual recommends?

My hydra-boost has been leaking out fluid since I bought my truck, I'm going to rebuild it probably in the next couple of weeks. Because of the leak I have had to fill my power steering fluid every three weeks. I have always used automatic trans fluid any Dexron kind. If your pump doesn't cavitate when you turn then your not doing any damage.

THX, I happen to have some Dexron III left :)

The fluid level is about 1.5mm under the low cold mark on a cold engine. I think it's very close to what it should be, so I didn't do anything about it yet. I think I will fill in some Dexron III to get to a more appeasing level and check it more often.

How would I know if the pump cavitates? Will it make some noise?

Yes, it will make an ugly noise, and with the noise will come difficulty turning the steering wheel. Hydraulic pumps don't like trying to pump air, and they let you know it.

If your manual states power steering fluid use power steering fluid. If it states ATF use ATF. Power steering fluid is a different additive package and a different base oil than ATF.

If you are not leaking any fluid just buy the plain power steering fluid witout stop leak.

Some brands available here in the US are STP, Valvoline.