On engine oil marked 5w-30, why is the latter number higher than the first. According to engine oil manufacturers sources, the numbers indicate viscosity. 5W indicates a cold engine (winter) and 30 at normal operating temperature. But the viscosity of oil drops when heated, so why is it a higher number?
In: Engineering
They’re two different scales. A theoretical 20w-20 oil wouldn’t mean it’s the same viscosity when it is cold as when it’s hot. Within the same grouping lower numbers are more viscous so 5w oil is thinner when cold than 10w is.
I’m not really sure why they use different numbers. My guess would be using the same scale for cold and hot would result in a huge gap in the middle
The way this type of oil works is that it has additives that stabilize the viscosity of the oil. At lower temps they resist the natural thickening of the oil, while after it heats up, it resists the thinning effects of heat. 5w-30 is a fairly common oil used for northern-ish areas that have winter but also get hot summers. Go further south to areas that are warm year round and you’ll see people using things like 15w-40 or even 15w-50.
It essentially means that, when cold, this oil behaves like cold 5-weight oil. When hot, this oil behaves like a hot 30-weight oil. THink of this as a “scale of runniness”. A set of numbers describing how runny this oil is. That means at low temperatures, it will still flow well because it behaves like a low-weight oil. It also means that, at high temperatures, it will still be able to maintain oil pressure because it behaves like a heavier oil. In short, it will tend to stay about the same “runniness” over a much broader range of temperatures than a single-weight oil.
Engine oil used to, and still sometimes does, have one number indicating the oil viscosity, because it was basically just dino juice pumped from the ground at that point, the simplest oil possible. This has a certain viscosity at various temperatures, based on the weight of the oil (literally weight per quart, this used to be checked by using a hygrometer to test density).
The things is, a thick, heavy weight oil that’s good when the engine is hot on a hot day as it doesn’t get too runny, also means that during a cold start you’d be starving the engine of oil for quite a while until it heated up. And a thin oil for getting your engine started during a Minnesota winter wouldn’t protect the engine once it heated up.
So the lubricant industry started making additive packages that they’d add to oils to keep them runny when cold. A 5W-30 starts out like a 30 weight oil, and then has additives to ensure it doesn’t become molasses during the winter. This still isn’t enough in all cases, and oils like 0W-10 exist for when it gets so cold out that your engine never warms up enough to use a 20 or 30 weight oil.
This improvement in lubrication, among others, is a big reason that engines have gained almost 10,000 miles in average durability per decade ever since the 1950’s. A new in crate 1950’s engine would be expected to have more than double the life it would have had if installed in the 1950’s, almost all down to lubrication.
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