Quote:
Originally Posted by bobcowan
Remember that street oil has two viscosity ranges, like 5W-40. When the oil is cold, the viscosity is low and the oil is "thin". Do you really want to be driving around with an oil that thin? Unless you're making a qualifying run, you do not. |
Perhaps it's the way I'm reading this, but you seem to imply that 5W40 is thinner when cold than it is when hot. Hopefully that's not what you mean.
The oil viscosity index of 5W40 indicates its 'weight' of '5' when cold (0°F), and its 'weight' of '40' when hot (212°F).
Explanation from
https://autosneed.com/oil-viscosity-and-weight/
The SAE classification for multi-viscosity oil consists of two viscosity grades, such as viscosity SAE 10W-30. The first part of the grade (10W) is the comparable grade of the single viscosity oil that features the oil’s weight at cold temperature.
The second part of the grade (30) is the grade of the comparable single viscosity oil that defines its viscosity at 212 degrees Fahrenheit. You should note that both parts of the multi-grade SAE classification are not viscosity values but grades.
Here is another interpretation of how to read oil viscosity (SAE 10W-30)
SAE = Society of Automotive Engineers
10W = the viscosity of the oil when measured at 0 degrees Fahrenheit (the “W” means winter grade)
30 = the viscosity of the oil when measured at 212 degrees Fahrenheit
In other words, SAE 10W-30 has a base rating of 10 when cold. Therefore, it will flow freely at temperatures as low as -20 degrees Fahrenheit. Usually, oil thins out when heated because it becomes thinner, its viscosity number becomes lower.
If you look carefully at the above chart, you'll see 5W oil has a Kinematic Viscosity of about 20 at 40°C (104°F), while a 40 weight has a KV of about 13 at 100°C (212°F). A 5W40 oil is equivalent to a 5 weight oil when cold (0°F) and equivalent to a 40 weight oil when hot (212°F) - but it's still MUCH thicker when cold than it is when hot - at least 50% thicker at 40°C / 104°F, never mind the much greater difference at colder temperatures.
A 5 weight oil, being much thinner when cold, will often do a FAR better job of flowing into thin spaces (e.g. bearings) and small orifices (e.g. hydraulic lifters). As such, a 5W40 oil is often a far better oil than 20W50 in order to properly lubricate an engine at startup - when most engine wear occurs. Here's a good chart, showing different multi-viscosity oils during warm-up:
Note the difference at 20°C (68°F) is much less than it is at 0°C (32°F), and the differences between 5W30 and 20W50 aren't easily discernible at 100°C (212°F).
https://www.kewengineering.co.uk/Aut..._explained.htm