Which Viscosity For Year-Round ATV Riding?

Update- September 2025– AMSOIL’s application guide suggestion for this Honda ATV model has evolved over the years. The suggested engine oils are now either AMSOIL ATV/UTV 10W-30 or ATV/UTV 10W-40. Either are great year-round options. However, a third option of AMSOIL Formula 4-Stroke 0W-40 Synthetic Oil still stands in our opinion. Formula 4-Stroke 0W-40 is a very viable replacement for either 10W-30 or 10W-40. It offers suburb winter perform and will not be a compromise in extreme heat.

A reader asks: I use my 2006 Honda Rincon ATV all year-round. It is used for pushing snow in the winter and for trail riding in the summer. The AMSOIL application guide suggests 2 different engine oil options for my model. It says I can use either the AMSOIL Synthetic 10W-40 ATV Oil or the AMSOIL Formula 4-Stroke 0W-40 Synthetic Oil. Is it possible to use one of these engine oils for both summer and winter?

Our Answer: Yes it is possible to choose just one of these oils for year-round use, rather than switch products seasonally. Either of these synthetic oils would be quite suitable for winter and summer service as they both flow exceptionally well in extreme cold. But as you run your ATV in winter, we would tend to lean to toward AMSOIL Formula 4-Stroke 0W-40 Synthetic Oil. Formula 4-Stroke 0W-40 has a cold pour point of -50°C (-58°F). The Synthetic 10W-40 ATV/UTV Oil has a very respectable cold pour point of -40°C (-40°F), but we would opt for the 0W-40 for that extra measure of cold-start protection. The AMSOIL Synthetic ATV/UTV 10W-30 has a cold-pour-point of -42°C (-44°F).

But is it OK to use a 0W-40 ATV oil in the summer heat? It’s more than OK. Remember that AMSOIL Formula 4-Stroke 0W-40 is still a 40-weight oil at operating temperatures. It is also test-proven to be exceptionally shear-stable. This means that it resists thinning out, no matter how hard you thrash your engine in the summer heat. Below is a chart illustrating how well Formula 4-Stroke 0W-40 Synthetic Oil maintains its viscosity compared to Polaris Synthetic PS-4 Plus and Arctic Cat Synthetic ACX in ASTM-7109 Shear Stability testing. Both the Polaris and Cat oils are 50-weight engine oils and each lost a third of their viscosity in the test process. AMSOIL 0W-40 stays steady and maintains its 40-weight viscosity.

So while either the AMSOIL 0W-40 or 10W-40 would be outstanding for all-season engine/transmission service in your ATV, we give the nod to the Formula 4-Stroke 0W-40 Synthetic Oil for its slight advantage in extreme winter cold.