Motor oil as well as other lubricants made from crude have many different sized molecules.
Premium Amsoil Synthetic, typically made from natural gas, has molecules all the same size.
When heated the former tends to evaporate off its smaller molecules and leave a sludge coating. Amsoil Synthetic has the opposite effect. It gradually removes sludge.
If conventional oil has been used for over 30,000 miles it might be best to do Amsoil Engine Flush when converting to Amsoil Synthetics.