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- Jun 3, 2021
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I think what confuses people about this is that we tend to envision what the oils flow/feel like at room temperature.I took this from the Castrol webpage. Maybe they oversimplified?
If you need a 15W-40 diesel oil, Castrol® GTX® Diesel helps extend engine life, protecting against the wear and tear of severe weather, rough roads and heavy tows. As a 15W-40 viscosity grade, it flows like a 15 weight oil from start up in Winter, but provides the protection of a 40 weight once the engine reaches full operating temperature.
Flowing "like a 15 weight in winter" means that it flows like a COLD 15 weight, which is still thick. It's just less thick than a 40 would be when cold.