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Originally Posted by x808drifter
You keep throwing that graph up here but I have to ask where it's from.
Who made it?
The only other place I can find it is subaruoutback.org in a single post.
No reference to who came up with these numbers.
I also can find no other info online backing any of these numbers up.
Or for that matter, how thick any grade of oil should be above operating temps, let alone 270F.
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Found searching on 0w20 5w30 viscosity vs. temperature. It's just an illustration, the point is the same whether those curves are perfectly accurate or not. Different specific oils may have somewhat different curves, but the general trend is the same. Higher temperature, lower viscosity...
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Until then I am of the thought, run what the manual says until an oil analysis says otherwise.
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The point is that *any* oil is going to thin out as temperature goes up. At 270F, if you want your oil's viscosity to be closer to what the factory-fill 0w20 is at normal operating temps (~190F-220F), you'll need to run heavier oil. This point is true whether or not the curves posted are 100% accurate. A 30-weight oil is going to have higher viscosity at 270F than a 20-weight, and 40-weight higher still.
What the manual specifies doesn't really apply if you're tracking at 270F+ oil temps. But some manuals still encourage use of thicker oils at higher operating temps...