Is there really any benefit or downside to using heavier-weight oils in summer months and lighter-weight oils in winter? Some of my older friends swear it's essential to proper maintenance.
Oil is subjected to more misinformation, controversy, out-of-date knowledge, and myth than just about any other aspect of car maintenance. Using the right oil is an essential part of keeping your engine healthy, but what does that mean?
Let's tackle the specifics of your question first. It used to be that changing oil weight for summer or winter months was part of proper maintenance. Old conventional oil formulations had only one viscosity, and oil would thin out as it was heated. In winter months this caused starting trouble because the oil would turn to molasses and the pumps couldn't lube the engine properly. To combat this, a lightweight oil such as 10-weight was used for cold weather, so it would flow, while heavier 30- or 40-weight oils were best in summer months to prevent the oil from breaking down in the heat. This problem was solved with multiviscosity oil, oil that flows better when cold, then thickens and protects better when it's hotthe best of both worlds.
With an oil like a 10W40 (the W stands for winter), the oil flows similar to a 10-weight in freezing temperatures to minus 30 C and protects like a 40-weight at 100 C. With this innovation in oil performance, changing weights for the season is no longer necessary and may be detrimental. Modern oils are very effective across all temperature ranges, and new engines are designed and tested to work specifically with only the type of oil listed in your owner's manual. Older cars can use modern oils too, just base the first viscosity on your climate, e.g., 0W for northern Canada, 10W for Florida, and use the original oil spec for the operating weight. Most older cars work fine with 10W30.
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