Synthetic is not some miracle drug, its just oil. You can use it anytime, mix it, before, after & during.
Yup, I've used synthetic on two preivous cars, which I bought new. On one, I switched to synthetic after it had 60K-80K miles; on the second, I switched to synthetic after the break-in period. Doing the same with the Celica now. Neither of those first two cars ever used oil; the last had 175K miles when I sold it.