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Motor-Oil Viscosity: A Slippery Subject

The oil viscosity your car needs is usually determined by your car’s manufacturer, but is there ever a reason to switch? Groupon sought out the answer.

Whenever a vehicle is in motion, engine oil has a lot of work to do: reducing friction between moving parts, keeping the engine from overheating, preventing components from prematurely wearing down. But there are many things that can keep oil from staying on top of its workload, and that’s where viscosity comes into play. Factors such as varying starting and running temperatures, an aging car, or heavy loads on board can all affect how motor oil performs. Fortunately, auto manufacturers specify what viscosity level is best for their vehicles, so most drivers never have to spend much time pondering this subject. But for those who are interested or who have to make elevator conversation with a talking car, there are a few basic principles to consider.

Like most liquids, when oil is cooler it’s more viscous (that is, slower-flowing), and when it’s warmer it’s less viscous. Higher-viscosity oils are most appropriate for cars that operate at higher temperatures or carry heavier loads. Lower-viscosity oils work better at lower temperatures since they don’t need to be heated to flow, but they generally offer less protection against wear.

There was a time when people who live in four-season climates would use one oil with a higher viscosity during the warm months and another during the cold months, but multi-grade motor oils solve that problem by working efficiently in both conditions. When you see a viscosity grade on a bottle of multi-grade motor oil—say, 10W-30—there are two numbers to consider. The number preceding the W stands for “winter” or “woolen-underwear season,” which reflects how the oil flows when starting an engine on a cold day. The number after the hyphen is the viscosity at 210°F, the standard temperature of an operating engine. (On both sides of the hyphen, a higher number means a higher viscosity.) What makes this type of oil so flexible? It’s the polymers—particles that expand as the oil heats up and slow down the rate at which the oil thins as it gets hotter.