Typically the engine manufacturers make a specific recommendation with their engine on how many gallons of petroleum oil premix to burn before you switch to synthetic. One of them cautioned their customers against using synthetic oil during break-in because they were having a problem with the cylinders losing compression. One cylinder would quit firing on a twin because of low compression. They found that if they took the engine apart, soaked the cylinder and piston in acetone to remove all traces of the synthetic oil, they could get the compression back by running the engine on petroleum oil premix.