Isn’t it amazing how some people will cling to old myths?
You can switch back and forth between conventional oil and synthetic oil as much as you want (as long as your car doesn’t require synthetic oil), and it won’t harm a thing.
I work with a mechanic who insists my car and motorcycles will run better on high octane fuel, even though none of them require it and they all run well on 87 octane fuel. I tried to show him Tom and Ray Magliozzi’s webpage outlining when high octane fuel is required and when it isn’t, and he wouldn’t even read it, saying, “You’ll believe anything you read on the internet.” He wouldn’t stick around long enough for me to explain that Tom and Ray are MIT graduates who ran their own auto repair shop for decades. It shows you how likely we all are to stick to our beliefs, even when presented with evidence that the issue isn’t faith-based, but factual.
My personal take on the synthetic oil issue (for cars that don’t require it) is that you shouldn’t leave it up to chance. If you think your car might benefit from using synthetic oil, why not find out to be sure? If I really wanted to know, I’d get a used oil analysis to find out rather than leave it to guesswork and people’s opinions.