I'm going to change my oil soon, and theres a lot of talk about oil weight here, but I don't understand what the numbers on the oil jug mean. Is the first number like 10w the weight when the oil is cold or at engine temp and the same for the other number? I was thinking of using shell 5w-40 instead of the normal 15w-40 for the winter. thanks for the help.

 

You basically have it right. The 15W means that at low temperatures (i.e., at cold startup), the oil will flow like an SAE 15 oil, while the 40 means that the oil flows like a 40 weight oil at 100 degC (212 degF), its assumed hot operating temperature.

Rusty

 

The temperature that the multi-viscosity oil reaches the higher weight is 140 degrees F.

 

Do a little Google searching on "SAE viscosity motor oil temperature". It used to be 210 degF for many years - it's now been metricated to 100 degC, or 212 degF.

Take a look HERE for an example.

Rusty

 

MHO is to use 15w40 to 10 degrees and if the temps will get below 10 degrees before the next change then use Synthetic 5w40. Dodge for 06 shows to use 15-40 down to zero and 5-40 for anything below zero.