Specific gravity refers to mass and density, usually in comparison to water.
I'll use metric to demonstrate, cause its easier...!
1 liter (about a quart...) of water weighs 1 kilogram (2.2 lbs) and has a volume of 10x10x10 centimeters. (3.93 inches)^3.
One can say that the specific gravity of water is 1.
Fill that same size container with, say M1 15W50,
and you will also have one liter of oil...BUT it will not WEIGH the same as the water.
Oil is lighter than water; the .875 refers to the weight compared to water...0.875 x 1kg = 0.875 kg, about 1.93 lbs.
5W20 Pennzoil = Sg 0.85
15W50 M1 = Sg 0.864
The difference in these two very different grades of oil is minimal.
[QUOTE]Originally posted by geeeman:
[QB] Specific gravity refers to mass and density, usually in comparison to water.
-*-* okay, but does it in any way play a role in how much additives the oil will take or how clean it has the potential to clean or how hot or whatever... eeek, what if any does this play a role in oil performance?
Some of your additives are metallic soaps and likely raise the SPG. Esters usually have a higher SPG than the parafins. A higher SPG might indicate more of those components, but the manufacture's specs would be a much more reliable guide. Since the SPG range of different oils is fairly close and the oil is flowing under pressure, I don't think the SPG itself has much effect on oil performance. When comparing the SPG of 2 different oils, make sure both are specified at the same temperature.
Specific gravity varies with additive content and base oil.
In general synthetics are lighter. I have oils that range from .931 to .824
An 85w140 is heavier than a 80w90 with the same additive package.
An ISO 220 mineral gear oil will be around .88 while a synthetic one will be around .85
I don't think it has much to do with performance, but affects how many liters you get if they sell it by the kilo. (as does Chevron)