Viscosity for dummies?

Not open for further replies.
Jan 27, 2003
My understanding of viscosity quite weak. After using the search button a bit, it's still pretty weak so here's a few questions.... Does a 5W-40 oil for example, actually thicken as it's temperature increases or does it just not thin as much as say a 5w-30 at the same elevated temperature? Does a straight 30 weight oil maintain it's 50 degree "viscosity" at 200 degrees? Does a 0w-40 have the same viscosity at 200 degrees as a 15w-40 at the same 200 degrees? Very much appreciate the help.... [ April 03, 2003, 10:57 PM: Message edited by: Ron-Indy ]
I'm no expert myself, but here is what I have learned so far. All oil thins as it gets hot, so any 30 weight oil will thin to the same relative viscosity when hot. This means a 10w30 would be a 10 weight when cold and would thin out to a 30 weight's viscosity when at operating temp. The difference in the 10w30 vs. the straight 30 is that while both are about the same viscosity hot, the straight 30 is a much thicker when cold and would not be good in a cold climate. The 10w30 allows it to have the 10 weight cold viscosity to aid cold starting and quick cold engine lubrication.
Ron, Here's the viscosity spec chart CentiPoise is a measurement of viscosity that's used for the cold measurement. CentiStoke is a different measurement of viscosity that's used for the hot (212°F) measurement. As you can see, there is a range of acceptable viscosities for 40 wt., for example. So, yes, 5W-40, 0W-40, and straight 40 wt. are the same viscosity @ 212°F, within the spec range. All oils get thinner when hot. A straight wt. oil does not have a cold viscosity test, so we don't know how thick it is when cold. Ken [ April 04, 2003, 12:56 AM: Message edited by: Ken2 ]
So to use basic terms, (need to spend the weekend on understanding the technical ones) a 5w-30 and 5w-40 have the same viscosity at 0 degrees but at 212 the 5w-40 is thicker than the 5w-30 but THINNER THAN IT WAS at 0 degrees or said another way, the higher the multigrade weight, the less it thins at higher temps, not the more it thickens. What's driving these questions somewhat is the filter pressure drop experiment that Bob is doing. Since he's running the tests at 70 degrees, would we expect to see higher pressure drops at colder temps and lower drops at operating temps? Thanks again
If I remember right, the low comparison point on a 5w oil is at -30C (-22F), and a 10w at -25C (-13F). You are dead on in your statement. At the cold temp, the 5w30 and 5w40 will be similar (but not the same since the 5w is a range, not a specific point), and at 100C (212F) the 5w40 will be thicker than the 5w30, but thinner than it was at cold temps. In other words, the 5w40 thins less than the 5w30 as it is heated. If I recall correctly, one of the next iterations of Bob's oil filter test is to use oil heated to around 200F to see how that affects the observations.
Not open for further replies.