Will changing motor oil brands, have an ill effect on my cars engine?

I just bought a 2007 Toyota Solara from CarMax. It’s a super-nice car with all the bells and whistles and only 43,000 miles on it. One mechanic tells me to never change from regular motor oil to syntheic motor oil, unless it’s a brand new car with 0 miles on it. According to this mechanic, it’s okay to use whatever motor oil I please, except the synthetics. Question: anyone had good or bad experiences with changing motor oil brand name, in your car’s engine? Please explain….........