Hi Mr Mikhail,
Don't worry about your words.It is OK.You may please note Mr Prabhakar Agrawal's post ,he was looking for the rejection limits of a 15w40 oil.So anything below and beyond the above mentioned limit (12.30 cSt - 16.30 cSt) will surely change the grade to either 30 or 50.That means we can not call it as a 15w40 oil.The drain out limits will be different for different engines as per the engine manufactures recommendations.That is why CUMMINS has recommended a decrease of 1 SAE viscosity grade.Got it.