Mixing weights - what do i get?

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Mar 21, 2005
Elderly County, Florida
Good afternoon fellow oilers: Quick question - if I mix a quart of 30W with a quart of 40W, will I then have two quarts of "35W"? Likewise, a 20W mixed with 40W would that make two quarts of 30W? Thanks in advance.
Originally posted by GreeCguy: Good afternoon fellow oilers: Quick question - if I mix a quart of 30W with a quart of 40W, will I then have two quarts of "35W"? Likewise, a 20W mixed with 40W would that make two quarts of 30W? Thanks in advance.
The API Engine Oil Classification Chart only gives "w" weights up through 25w. The weights represent a grade category with a range of viscosity (see chart). A 30 weight ranges 9.3 to 12.5 cSt at 100C. A 40 weight ranges 12.5 to 16.3. If you mix 30 weight with 40 weight you will either get a thick 30 weight or a thin 40 weight, but it will be somewhere in between. You can calculate it on this Calculator (just put dummy values in for the 40C to make it work). The 100C viscosities are available at the manufacturer's web site ( Valvoline Product Data for example). Or there is a pocket calculator formula. In any event the mixed viscosity is a bit below the arthmetic mean of the two viscosities. Edit: Here is the equation behind the fancy blending calculator Gary and I posted: Log Vs = (n log V1 + (100-n) log V2) / 100 where Vs = viscosity sought n = percent by weight of the first component of the blend having a viscosity of V1 V2 = viscosity of the second component of the blend. You can easily expand the equation to handle three oils in the mix. Happy mixing:  - Oh yeah, if you want to explore oil mixing, check this thread out. [Freak] [ April 27, 2005, 03:24 PM: Message edited by: TallPaul ]
Therefore, to quote Captain Ahab in his quest for the white whale: "If my calculations are not faulty . . . " a 30/40 blend - 12.6 cst@100c - (a heavy 30W) a 20/40 blend - 10.8 cst@100c (a light 30W) a 15W40/5W20 blend - 10.66 cst@100c 70.4@cst@40c - (10W30) a 20W50/10W30 blend - 15.4cst@100c 118.3cst@40c - (15W40) a 5W20/20W50 blend - 13.1cst@100c 91.2cst@40c - (10W40) And who was it that said: "Give my creation Liiiiife!!!!!" [Freak]
12.6--close enough to a heavy 30 wt, but actually just a hair into the 40w class. Of course the margin of error may be bigger since this equation/calculator was meant for base oils before viscosity index improvers were added, so we may be off, but close enough for our purposes I would suppose. A 10.66 - 10.8 cSt actually is pretty typical of off the shelf 10w30 oils. We probably should drop the hundredths since that is beyond likely accuracy given the variability in the manufacture and the above limitations of the equation, so 10.7 is close enough. Yes, a 20W50/10W30 blend most likely would end up a 15W40. Want to try a 15w30? Maybe 2 qts straight 30 and 3 qts 10w30. Frankenstein's oil mix [Freak] : I once had a wild brew in the wife's Aerostar, including Maxlife, Durablend, Havoline, Royal Purple (almost typed Deep Purple, hee hee), and Rislone. Mostly 10w30 and 10w40 but the Rislone is more like a 20 weight. And instead of just pouring the different oils into the engine for the engine to mix, it is a lot more fun to play Dr. Frankenstein by saving a 5 qt oil jug so you can mix them in the celler (or attic) on a dark and stormy night. [Freak]
Originally posted by GreeCguy: My wife is going to hate you!
Oh great! Now two women hate me for playing with oil. [Big Grin] Yeah, once you mix it up it seems you can't stand to let it just sit in the jug. Gotta use it. I usually start my mixing a week or two before the anticipated oil change.
My wife is going to hate you! For years she hasn't been able to find any of her measuring cups as I've been using them to mix different brews. Up until this point, I've been doing it without really knowing what I was doing - just guessing. Like Tevia it was always, "A little bit of this, a little bit of that . . ." I've got a bench set up in the shop with measuring cups, jugs and bottles, all labeled - "75%40W/25%Marvel Mystery Oil" "10%Rislone/90%10W40" etc. Now. . . . "let the true mixing and labeling begin!!!" [Eek!]
This is torture! I just made a gallon of 15W40 and I just changed the oil in all my vehicles LAST SATURDAY!!!! (I think I'll take a detour on my way back to work - like through Canada).
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