Oil Viscosity @ 40'c ?

So all oil is measured under the same conditions.

There are other standard temperatures too.

Oil Viscosity - How It's Measured and Reported

It all depends on the use to which you want to put the oil. For example, summer rated motor oil is tested at 100° C, but winter weight oils are tested at a variety of colder temperatures. If you lived in International Falls, and you wanted your truck to turn over on a - 28° F morning, you really wouldn't care what the oil did at 100° C. So, different standards bodies have different temperatures for testing.

What kind of oil are you asking about, and what is the governing standards body?

Check with ISO 3448.

"Can we measure viscosity at 20'c?" Yes, of course you can, if that is your performance criteria.

"But I didn't understand that 40'c is it any standard?" Yes, it is a standardized testing criteria for reporting, a temperature that is common. Many industries experience different temps in actual use, and it is wise to investigate potential products at anticipated temperature ranges that you may experience.