What makes an oil a 5W30 or a 0W30?

[N0DIH]

I just put some values in a spreadsheet based on datasheets I pulled up from 2 oil manufacturers. It seems with Mobil 1, the plain 10W30 is actually a THINNER oil at 40C (the cold temp) than the 0W30. What? What gives?

Seems though, Amsoil is still top dog in the thinner at cold temps, significantly more than Mobil 1. I found as many as I could, so here is a fairly comprehensive list.

I will have to keep that in mind for my oil change I am about to do....

With all the cold weather we have been having (-16F!) this is a big deal, make it as free flowing as soon as possible.

[Benjamin Simon]

I don't know enough about oils to even understand what you are talking about.

[carguyshu]

i think it is the flow rate, rather than the thickness that determines it...could be wrong

[N0DIH]

It is a viscosity test, drop a ball through the oil in a known dimension tube with the oil at a specific temp and measure the rate it drops.

So the point is some 0W30 or 5W30 oils are actually thicker than 10W30 when warm, but often only slightly thinner than a 10W30 when cold.

What I do is just run a 10W30, as it still seems to the the superior oil for viscosity stability.