There seems to be some confusion as to whether AMSOIL Formula 4-Stroke® Power Sports Synthetic 0W-40 Motor Oil (AFF) can replace 50 weight oils like Polaris Synthetic PS-4 Plus and Arctic Cat Synthetic ACX. The Polaris product is a “2W-50” (of all things), while the Cat oil is a 5W-50.  So how can a “0W-40” offered by AMSOIL replace these 50-weight products?  AMSOIL tested all 3 products using the ASTM-7109 testing protocol for shear stability. In the chart below, you’ll see that after just 30 passes, both the Polaris and Cat oil had thinned to the point that they were already out of grade. After 90 passes, the Polaris oil lost 33% of its viscosity, while the Arctic Cat product lost 30%. AMSOIL AFF lost just 5%, leaving it perfectly “in grade”.AMSOIL 0W-40, Polaris & Arctic Cat Shear Test

So why does AMSOIL stand up so much better than the OEM products? While we know from experience that AMSOIL delivers bulletproof products, we can speculate as to why the Cat and Polaris 4-stroke oils did not stand up well. Again this is speculation, but we suspect Cat and Polaris used generous amounts of “viscosity index improvers” in their formulas.

“Viscosity  Index Improver” definition from Machinery Lubrication:
Viscosity modifiers are polymeric molecules that are sensitive to temperature. At low temperatures, the molecule chain contracts and does not impact the fluid viscosity. At high temperatures, the chain relaxes and an increase in viscosity occurs.

Another way to describe this chain is in comparison to a slinky, a lazy-spring coil-shaped toy. Working in a way similar to the people in a hallway analogy, the slinky contracts when cold and stretches out when hot. When contracted, the molecules flow past one another easily, but when extended, they get caught on one another and impede the flow of the fluid they occupy.

Keep in mind that as temperature increases, the viscosity decreases. The addition of modifiers will only slow down the rate at which the viscosity decreases.

The downside to viscosity index improvers is that they can be quite susceptible to shearing, thereby leaving the motor oil vulnerable to thinning at high temps. They can make up a noteworthy portion of some motor oil formulas, but offer nothing in the terms of wear control or friction reduction. Their sole purpose is to expand as the oil temperature rises, preventing the oil from thinning out.

Now back to our speculation. The Polaris oil is a “2W-50” and the Cat oil a “5W-50”. The “W” number signifies the winter viscosity. Under cold weather testing parameters, the Polaris oil acts as a “2” weight and the Arctic Cat oil is a “5”. Both act as a 50 weight at 100C. In both cases, the stretch from a “2” or “5” when cold up to a 50 weight (when hot) is a long one. Unless they are using ultra-sophisticated basestocks and additives, chances are high that these oils are loaded with viscosity index improvers and perhaps that’s why they shear down out so readily.  Again this is our guess, but these test results certainly seem to back our suspicions.

So why doesn’t’ the AMSOIL AFF 0W-40 thin out under the same testing? After all, isn’t the stretch from “0W” (when cold) to a 40 weight (when hot) a significant one as well? Yes it is, but AMSOIL AFF does not contain any viscosity index improvers, just a very sophisticated synthetic base and powerful additive package. It contains no fillers like vi improvers, so it is much more resistant to “shear-down”.

So which oil would you rather take into the heat of battle in your ATV engine?