writing on oil cans

[herbwhite59]

What does 10w30 mean on oil cans? Is it 10 winter and 30 what? And how is it determined?

[Frank]

Was going to type this but easier to clip from google hit:

The first number '10W' is the viscosity of the oil at cold temperature and the second number is the viscosity at 100 °C (212 °F). The two numbers used are individually defined by SAE J300 for single-grade oils.

As this chemist understands it, there are polymers in the oil that lengthen with temperature causing the viscosity increase.

[CRNG]

On Sat, 17 Feb 2018 11:53:34 -0800 (PST), [e-mail] wrote in

The viscosity of the oil at operating temps is 10, but it's lubricating effectiveness is 30.

[Clare Snyder]

I'll try to make it simple and answer the questions raised in some of the replies.

First of all,ALL oil gets thinner when it is heated up, so no, it is not 10 weight when cold and 30 weight when hot.

Close though.

The first number (10w) means it "behaves like a 10 weight oil at low temperatures" when it is cold, and the 30 means it "behaves like 30 weight oil at high temperatures" when at high temperatures.

Hot 10W30 oil is still thinner than cold straight grade 10

The temperatures and test schedules are specified in SAE papers

Compared to a 10 weight straight grade oil, 10W30 does not thin out as much when hot. Compared to a 30 weight straight grade oil, it does not thicken as much when cold.

In a standard (non synthetic) oil this is accomplished by adding a significant amount of "long chain polymer" viscosity index improvers that coil up and uncoil under influence of temperature change to thicken the oil when hot, so when those molecules "shear" ofer time, the oil loses hot viscosity. This is one reason oil needs to be changed, even if it is still "clean"

Synthetic oils, by their very nature, have a better "viscosity index" so synthetic oils use much less, and different, viscosity index improvers, and their viscosity does not change nearly as much over time/usage as conventional multi-grade oils - and therefore can last longer between changes.

Gear oils are also available in "straight grade" and "multigrade" - ie SAE 90 and SAE 75W90