I recognize that there is a risk associated with starting another oil
thread but I think I can add something to the subject rather than
rehashing the same old info. I got to thinking - "If Mobil can blend up
oil, why can't I?" That led to further wonderings along the lines of
"How close to 0W-40 or 5W-40 can I get mixing 15W-50 and xW-30?" Here's
what I've learned.
There is a well known rule that says if you mix oils of different
viscosities the log(Vis mix) = Frac A*log(Vis A) + Frac B*log(Vis B).
It's well known but tough to find references for on the web.
You can calculate viscosity at another temp if you know the viscosity at
2 temps using the equation
log(log(Vis+0.7)) = A - B*log(T-273).
You have to solve for A and B using known reference points such as the
40 and 100 deg C data published on the Mobil 1 web site. See referenced
spreadsheet. I'm sure there is more elegant solution than my trial and
You can calculate the Viscosity Index of a blend if you know the
visosity index, viscosity at 100 deg C, and viscosity at 40 C of an oil
with the same 100C viscosity as your blend. See the spreadsheet for
details. The formulas don't lend themselves to text.
You can get an exact viscosity match for Delvac 5W-40 using 28% Mobil 1
10W-30 and 72% Mobil 1 15W-50. I make no claims about TBN, HTHS or other
You can get reasonably close to 0W-40 using about 33% or xW-30 and 67%
of 15W-50. The 150 deg C viscosity is within 0.3 cP when the 100 deg C
viscosity matches exactly. See spreadsheet referenced below for details.
I didn't project cold weather properties of the blend but it will
certainly be better than 15W-50 and worse than 0w-40.
There is some interesting references to SuperSyn on the Exxonmobil
Chemicals website. Seems that they sell SuperSyn blending stocks. They
appear to be higher molecular weight versions of the standard synthetic
Here's a link that has a lot of basic info on viscosity. It's the only
reference I found with useful formulas for ASTM D 2270 Viscosity Index
calculation for V.I.s over 100.
Here's a link to the spreadsheet I worked up.
Oil Blending Spreadsheet
Anyone see a downside to blending?