I changed back and forth a couple times on my GT, it has 196K miles on it. I don't think it hurt it.

Consider that myth busted.
Synthetic oil can go a long time without a change, just keep it clean by changing the filter every 3-5K miles. I read a study where they found out that most of the engine wear occurs in the first 3K miles after you change the oil, then it levels off.
In this study, they took out 1 quart or maybe it was .5 quart every 1K miles and sent it to get tested. They went on like this until 18K miles with an oil filter change at 8K miles. The test was with Mobil 1 in an LT1 camaro and they stopped at 18K because they wanted to try the test with RedLine.
In my opinion, sythetic oils are worth the switch at any milage, just make sure to keep it topped off, that's the most important thing.