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Paul

Sorry but your math is wrong. Oil viscosity doesn't follow a linear relationship with temperature, so your estimates for viscosities at 0°C are way too low. In your examples in the text, the Havoline would be about 2200 cSt and the Rotella T 15W-40 around 1500 cSt at 0°C.

I would also argue that you need a thicker oil at start-up because the clearances and tolerances in the engine are larger until it heats up and everything reaches its operating temperature (and therefore size). A thin oil at startup might not protect bearings or seal piston rings etc. Of course there is a compromise between flow rate and oil film thickness; you want lots of both ideally but they are often inversely proportional.

Finally, you suggest an iso-viscous fluid would be ideal (ie one that is the same viscosity at all temperatures) and mention that 15 cSt would be best. Wrong again - at piston ring zones the temperatures can reach 200 - 300°C, where the oil gets to a pretty watery viscosity (your Havoline would be about 2.6 cSt and the Rotella T 15W-40 about 3.0 cSt at 200°C). Note first that the multigrade is now thicker than the monograde when hot, which is what you want, but also note the low numbers. Engines are designed to have oils around this viscosity when at working temperature. An oil that was a constant 15 cSt would be too thick, meaning it wouldn't flow into the piston ring zone very well and would really hit fuel economy.

Furthermore, using kinematic viscosity in this way is misleading because under certain conditions the oil experiences a great deal of shearing, which lowers its effective viscosity. This is where the High Temperature High Shear (HTHS) property of an oil becomes important.

Blake Sobiloff

Paul, thanks for your comments. I used the viscosity calculator at http://www.eskoindustries.com/viscadv.php, which purports to use an Andrade correlation to influence its calculations. I'm curious to hear how you came up with your numbers--I'd love to have a more accurate tool.

The thick vs. thin debate is a long-standing one. Personally, I prefer a thin oil that will quickly establish a hydrodynamic boundary and distribute heat, but I don't think there's conclusive evidence for either viewpoint at this time.

I'm fully aware that some portions of an engine run hotter than others, and that temporary shearing occurs as well. I didn't want to explore all the corner cases of tribology as they detract from the main point of the post: No current oil is too thin at startup.

Jim Ruggles

There is a factor of oil you have not addressed,and that is the film strength and load carrying capacities. I think this is measured on a scale that goes as high as 3000..The higher the number,the thinner the oil can be with out making metal to metal contact..Such an oil does exist..You need to look at some of KLOTZ OIL..

Blake Sobiloff

Jim, there are plenty of oil factors that this post didn't address, but the article wasn't intended to be a comprehensive study, either. Film strength is the ability of an oil to avoid being displaced by a high load and is typically measured in PSI. KLOTZ is an ester-based oil like Red Line. The polar nature of esters gives them a much higher film strength than conventional oils, but more importantly in this context, ester oils (and PAOs) naturally thicken and thin less than conventional oils. This means that they will be closer to the ideal viscosity at startup, and during the first few seconds before positive oil flow has been established throughout the engine, their polar nature will help them prevent more startup wear (assuming equal EP additive quality).

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