Is a real Grayscale image always 8 bit per pixel indexed?
I don't think that it is that easy. Grayscale images do have certain benefits when you think of imaging experiments. When the measurement is squeezed into eight bits, even though the measurement is much more precise, then there is a loss of information due to the quantization. See the DICOM format for example that is widely used in medical imaging. It is very common there that the information is encoded in 12 bit, or even more.