I’m really not an expert in this area. I chose this oil as someone rebuilding an engine who was trying to avoid the horror stories he’d read of new cam failures of flat tappet cams with modern oils. I ran break-in oil for 20 minutes, Valvoline Race oil for the next 500, and put Valvoline Race oil in after that—700 miles on rebuild since startup in September.
Synthetic oils have a lot of advantages over conventional, and you’d likely be fine running in an established engine. When I tore my engine down, it was so filthy, worn, and the tolerances were so loose, I think the synthetic I had run in there was likely a waste of money.