Random Thought Thread

bluemax_1 said:
And here I thought the post was humor.

If it actually isn't:
A) For something that's spec'ed for 10w40, running either 5w40 or 15w40 is fine. If the first number is lower, that's sort of OK (***details below).

The first number is the cold viscosity. In theory, the lower the cold viscosity, the better, as the oil flows better on cold startup.

The 2nd number at the end is the hot viscosity, or the viscosity at typical running temperatures. That's the number that's important, as a lower hot viscosity may be too thin to maintain the oil film necessary for protection under load.

As far as the cold viscosity goes, in theory, the lower the better, but with caveats. Depending on the particular multigrade oil and formulation, some oils require more VIIs (Viscosity Index Improvers) to achieve the viscosity spread.

The viscosity spread used to be gauged by dividing the hot viscosity by the cold viscosity, eg. a 10w30 has a 3x spread. A 10w40 has a 4x spread, a 5w40 has an 8x spread (I say 'used to be gauged' because now we have 0w30 oils etc. and dividing by zero would give math teachers an aneurysm).

Anyway, to attain the viscosity spread, some oils need VIIs, which are basically long-chain polymers. Multigrade oils start out with their base viscosity (i.e. if you want to formulate a 10w30 oil, you start with a base oil that has a cold viscosity in the cSt (centiStoke) range for a 10 cold weight oil, then add VIIs as necessary to limit how much the oil thins when hot.

When cold, these long-chain polymers stay tightly coiled, allowing for the lower cold viscosity. When hot, the long-chain coils would only uncoil so far, limiting how much the viscosity thins out when hot.

Some engines stress the oils a lot more than others, and can result in a phenomenon known as viscosity shear. The long-chain polymers physically shear, and no longer limit how much the hot viscosity decreases. Viscosity shear can result mechanically (turbocharged engines pushed hard, towing very heavy loads, engine run extremely hot), or chemically as the oil degrades.

5w40 is a wide spread, so if the engine will be pushed hard, you'd want to make sure it's a good oil, or only use it for colder weather and lighter loads.

Conversely, I wouldn't worry about running 15w40 in the South Carolina summer. It's not cold enough for the cold viscosity difference to make a big difference and the smaller viscosity spread will maintain viscosity better if the engine is pushed hard. Just change it before the weather gets too cold.

For diesels, Rotella and Delo are both great oils.
Click to expand...

Finally! Someone around here who can hold their own with NtM talking science and physics and stuff.
 
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