dear i put fully synthetic oil in my almost 300k driven 19 year old ride. no issues. car has better milage better pick and better idling. its just that need to keep an eye on oil level since synthetics are more prone to get reduced over service interval so just need to keep them topped up appropriately.
fully synthetics are meant to ensure proper lubrication over longer service intervals and are more stable at higher temperatures and start stop traffic.
there no myth associated to old or new engines.
better oil will make engine perform better its just that plain simple.