There are lots of things people think will have an impact on their viva or their prep. Here’s a partial list of factors people think could make a difference, for good or bad:
Your examiners. Your institution. The number or types of awards or results. Whether you were part-time or full-time. Home or international. Number of supervisors. Time spent on your PhD. Do you have a Masters? How long between submission and the viva? Will you have an independent chair? Have you cited your examiners? Number of references in your bibliography. Number of chapters in your thesis. Number of published papers during your PhD. Number of hours spent on prep. Will you have had a mock viva? Have you read a book about the viva? Did you go to a workshop about the viva?
I’m a mathematician and there’s part of me which would love to take all of these variables and make an equation. But there’s far too many. Lots of them could make a difference.
So why not focus instead on the constants? The things that are always there.
You did the work. You made the choices. You steered yourself. Your examiners are capable researchers, who know what they’re doing in the viva. You are a capable researcher by the time you submit, and have the time to prepare yourself for the viva.
Focus on the constants that hold true for everyone.