[cashflyer]

Viscosity is viscosity.
Per the SAE, a 40 weight oil must flow at between 12.5 and 16.3 on the kenematic scale, whether it is dino or synthetic.



Something to think about when comparing oil brands and types:

Group IV (4) and Group V (5) base oil (synthetics) are chemically made from uniform molecules with no paraffin and don't need Viscosity Additives. However, in recent years Group III (3) based oils have been labeled "synthetic" through a legal loophole. These are petroleum based Group II (2) oils that have had the sulfur refined out making them more pure and longer lasting. Group III (3) "synthetic" motor oils must employ Viscosity Additives being petroleum based.

Group V (5) based synthetics are usually not compatible with petroleum or petroleum fuels and have poor seal swell. These are used for air compressors, hydraulics, etc. It's the Group IV (4) PAO based synthetics that make the best motor oils. They are compatible with petroleum based oils and fuels plus they have better seal swell than petroleum.

As more and more large oil companies switched their "synthetic" motor oils to the less expensive/more profitable Group III (3) base stocks it has become much easier to identify which are PAO based true synthetic.

Most, including Castrol Syntec, have switched to the cheaper/more profitable Group III (3) petroleum based "synthetic" motor oil.

AMSOIL Synthetic Motor Oils that are labeled as "100% Synthetic" are PAO based true synthetic motor oils. Unfortunately, even Amsoil has yielded to the rising costs of PAO base stock and those oils which are not labeled as "100% Synthetic" are often either Group III or blends.