Synthetic Oil in older tractors?

So I have started a side business of working on and restoring older tractors. I have put up some pictures on here in my gallery of one of them. I am getting a lot of people who want thier oil changed over to synthetic. Is it OK to run synthetic in a 4020 or 3020? Some people say it shrinks the seals, others say it will leak because its so much thinner. Other say its a myth. I would really like a good answer I can give my clients an feel confident that I am telling them the truth?