Thread: Idle oil pressure

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I know Denny is well informed enough to know how oil works, but I would like to clarify what he's saying for the sake of accuracy.............especially for those who might be prompted to do more research and might be confused by what they read elsewhere that might sound contradictory. ALL motor oils thin as they heat up, straight or multi-grade, doesn't matter. You prove this when you see your pressure reading at startup being a higher number than when the engine gets to operating temp (assuming you're reading at the same rpm). Pressure values are a measure of resistance to flow. The more viscous the the oil, the higher the resistance to flow, the higher the pressure reading. That's why most oil pressure questions that don't include noting the oil's viscosity aren't able to be addressed accurately.

To save me a lot of typing (keyboarding I guess would be more accurate) here's a decent article that explains what's at work for those that like more research; http://www.texacoxpresslube.com/carc...viscosity.html

But just to put a short, inconcise summary on it, oil vis is graded basically at two temperatures for a straight grade. 100f, and 210f (the article above is at 40c and 100c, essentially the same). If the oil flows through the test device at a certain, measured time standard it is graded accordingly. For example the same quantity of an SAE 20 will take longer, at the same temp, than an SAE10 to flow through the test device. Each successive higher grade does likewise.

Multigrades, designated by having two grading numbers, the first followed by a w (for winter, not weight as some mistakenly believe) measured at essentially 0 degrees f (actually done in a cold cranking simulator as referenced in the article), and the second being the rate of flow at 210f. So, for instance, a 10w40 oil has the characteristics of a straight grade 10 at 0 degrees, and the flow characteristics of a 40 at 210f. The idea being to give the quicker flow of the 10w at startup, and the "thicker" (in quotes because it's still thinner at the high temp than at low temp, but not as thin as a straight 10 would be) protection of the 40 at operating temp.