Great question, in the UK conversations with your boss are normally very informal, and if you were driving in a car and you wanted to the window open you might well ask them to open it. "can you open the window" is a little odd, "can" is really asking if you have the ability to open the window so you would use "would you please open the window?" as the best way.

Hi Rio! Another good question. In my experience it's always better to be polite. I only ever asked my managers for their signatures on paperwork , time off requests etc. Most of our interactions revolved around what I could do for them. Now, when I was a manager myself, I preferred to keep everything formal with other employees. I didn't want them to be too comfortable with me since it was my job to evaluate them. It also helped me to avoid awkwardness when I had to take disciplinary actions.