The best motor oil for most car engines depends on the relative age of the car. Synthetic motor oils, such as Amsoil Synthetic Blend 15W-40 Heavy Duty Motor Oil and Mobil 1 10W-30 Full Synthetic Motor Oil, are best for most cars, while older cars may require higher viscosity oils.
For cars considered older or "classic," one of the best motor oils is Valvoline Premium Conventional SAE 50W Motor Oil, which is a faster-flowing and thinner oil. It is important to note that motor oils can differ by brand. A local mechanic can provide an answer uniquely suited to a particular vehicle.