True, though I feel most cases involve ama & amaba. In this case, Si algún día yo tuviera la oportunidad, quiero visitar... The speaker 'comes back' from a far-off possibility, to express a current desire. But Si tuviera... voy a visitar... would be stranger in Spanish.The same is the case, I believe, in Spanish.
A friend said to me: "If I had the opportunity, I will travel to Mexico" and I thought, that was incorrect according to the conditionals rules...but is it possible to use "will" instead of "would"?
Yes, I said it was ungrammatical and mostly because I cannot imagine a native speaker saying it. Ever.It's a mistake, plain and simple. That some native speakers might say it that way changes nothing. Sometimes when we speak, we sort of change our minds in the middle of a sentence, resulting in a mismatch of tenses, etc.