Amsoil Signature Series 5W-30 Synthetic Motor Oil. Amsoil Signature Series Synthetic Motor Oil is engineered to outperform conventional and synthetic motor oils. It combines industry-premier synthetic technology with Amsoil premium additives in a unique formulation that exceeds the higher performance demands of modern engines. Signature Series delivers the performance and value upon which the Amsoil reputation is built. The culmination of more than 40 years of expertise, its exclusive formulation raises the bar for motor oil performance.