Hey everybody. First post. I've really been geeking out on motor oil for the last couple of weeks, and in the process I've been looking at viscosity specs of different oil brands.
After looking at the specs for pretty much all of Mobil, Amsoil, and Valvoline passenger car oils, I've come to the conclusion that the XW-XX rating of the oils really doesn't mean a whole lot. For instance, Mobil 1 0W-30 is essentially the same viscosity as Mobil 1 5W-30 at all temperatures. Many 5W-30's from other manufacturer's are actually thinner than the Mobil 1 0W-30 at low temps (0 degrees C). Why would they advertise it as a 0W-30? Likewise, Mobil 1 5W-20 Extended Performance is thinner than some 0W-20's at low temps, but with the same high temp viscosity.
The ability of an oil to resist thickening as it cools from the operating temperature is can be measured and is called the viscosity index. For two oils with the same viscosity at high temp, the one with the higher viscosity index is better since the oil will be thinner and easier to pump when it is below full operating temperature.
One would expect wide viscosity range oils such as 0W-40's to have a very high viscosity index, and oils such as 5W-20's to have a lower viscosity index. This is not what is observed, however. The 0W-40 will be significantly thicker than the 5W-20 at low temps, and at high temps it will generally be on the thin side of what is allowed for a 40wt. I suspect that the oil manufacturer's must do this because they just cannot make an oil with a high enough viscosity index to be a "true" 0W-40.
I'm sure many people on this forum are aware of these facts, but I found them interesting so I thought I'd share. Moral of the story? Before choosing an oil, check the viscosity ratings, which are usually available on the manufacturer's websites. And don't expect that 0W-40 oil to flow as well as a 5W-20 on those cold winter mornings.
Cheers
After looking at the specs for pretty much all of Mobil, Amsoil, and Valvoline passenger car oils, I've come to the conclusion that the XW-XX rating of the oils really doesn't mean a whole lot. For instance, Mobil 1 0W-30 is essentially the same viscosity as Mobil 1 5W-30 at all temperatures. Many 5W-30's from other manufacturer's are actually thinner than the Mobil 1 0W-30 at low temps (0 degrees C). Why would they advertise it as a 0W-30? Likewise, Mobil 1 5W-20 Extended Performance is thinner than some 0W-20's at low temps, but with the same high temp viscosity.
The ability of an oil to resist thickening as it cools from the operating temperature is can be measured and is called the viscosity index. For two oils with the same viscosity at high temp, the one with the higher viscosity index is better since the oil will be thinner and easier to pump when it is below full operating temperature.
One would expect wide viscosity range oils such as 0W-40's to have a very high viscosity index, and oils such as 5W-20's to have a lower viscosity index. This is not what is observed, however. The 0W-40 will be significantly thicker than the 5W-20 at low temps, and at high temps it will generally be on the thin side of what is allowed for a 40wt. I suspect that the oil manufacturer's must do this because they just cannot make an oil with a high enough viscosity index to be a "true" 0W-40.
I'm sure many people on this forum are aware of these facts, but I found them interesting so I thought I'd share. Moral of the story? Before choosing an oil, check the viscosity ratings, which are usually available on the manufacturer's websites. And don't expect that 0W-40 oil to flow as well as a 5W-20 on those cold winter mornings.
Cheers
