For the Honda Shadow VT600C, you need 2.1 liters for an oil change and 2.3 liters if you also change the oil filter. For the VT 750 Shadow, you need 2.4 liters, 2.6 if you change the filter. For the VT1100 Shadow Spirit you need 3.1 liters and 3.3 if you replace the filter.
If you change your oil regularly and keep up with all scheduled maintenance, there is no reason you can't get a Honda Shadow past +75K - 100K Miles. How you drive it will also play into that (don't race it). Remember, you take care of it and it will take care of you.
10-20 based on if you spill it alot. Above answer is better left out. Which Nighthawk? I think there is a 250 and a 750. '91-'94 CB 250: 1.4qt (1.3L) 10W40 Honda oil GN4. '91-'93 CB 750 : 2.9qt (2.7L) 10W40 Honda oil GN4.