Is there any restriction on type of pen to be used while writing Civil Service Mains examination?

No.

You should carry a black ball point pen to mark your OMR sheet and sign it. This is not applicable for the answer sheet.

There is no restriction on the type of pen that can be used. I have seen people using a wide variety including hero ink pens, gel pens, ball points etc.

The paper quality is good these days and ink pen also doesn’t cause any issue.However, I would advice that you stick with a ball point pen to be on the safer side. They also give a better flow of writing since you have to write so much in so little time.