Question about mixing different grades

Messages
77
Location
Florida
I am curious as to the results of mixing different grades of oil. I have searched but could not find a formula or a calc for determining the resulting grade after mixing oils. The following makes sense to me, but I want to make sure it is right. % of xW1-G1 + % of xW2-G2 = xW-G (xW1 * % of total) + (xW2 * % of total) / 1 = xW (G1 * % of total) + (G2 * % of total) / 1 = G Using the above equation, 3 quarts of 5W-20 and 1 quart of 15W-50 would yield an oil that is 7.5W-27.5 (5W * 75%) + (15W * 25%) / 1 = 7.5W (20 * 75%) + (50 * 25%) / 1 = 27.5 Is this correct? Am I calculating this correctly, or am I leaving something out? Thanks!
 
Messages
13,132
Location
By Detroit
For that online calculator, if you can't get the 40C number for some oils, you can plug bogus numbers into the 40c just to get it to work. The 100C result will be fine, but the viscosity index will not because it is tied to both numbers. The viscosity equation behind that calculator is that the resulting viscosity is the antilog of the weighted average of the logs of the different viscosities being mixed. Can do it on your pocket calculator if it has log function. BTW, the calculator is meant for mixing base oils, that is before viscosity index improver (VII) is added. Don't know if the VII will skew things, but probably it is close enough for what we are doing here. You could buy oil of the viscosity you want, but most 10w30 is 10.5 cSt at 100C and most 10w40 is around 14. If you want 12, there are a few such 10w30s (prob. mostly high mileage, which is fine), but you also can mix about 50/50 10w30 and 10w40 to get there.
 
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