# Equation for mixing oil viscosities

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#### ToyotaNSaturn

Can't find the old topic thread, so I thought I'd start a new one... I'd like to mix M1 5w-30 and 15w-50 and am looking for the equation to find the adjusted viscosity.

TIA
Mike

I don't think you can do more than average the numbers. The best way would be to mix it up and do a VOA and see what you have.

The guys at M1 told me that mixing was perfectly ok to do with any grade of their oil. And using some 5W30 to thin down 15W50 could be doen with no problem.

I did this for a few OCI's until the 0W40 became available. Now there is even the 5W40 SUV stuff out there.

FWIW- I found a "Two Components Viscocity Blending (cSt)" chart, page 12 of a 214(!) page download, labeled HANDBOOK-Echyl Corporation. I believe I found it through the BITOG forums, and I downloaded it 3/19/04, 1.30 Kb. Sorry I can't be more definite. If I can figure out how to save and post the page I will. Don't hold your breath, it's been uncooperative so far.

Awhile back (maybe December) someone posted at BITOG about how the viscosity of a mix of two liquids of differing viscosity would be somewhere lower than the arithmetic mean. Given that, I would guess equal parts 5w30 and 15w50 probably will get you a really thick 5w30 or a thin 10w40. Since I have heard M1 is on the thin side anyway, maybe you should bias heavily towards the 15w50 one or two quarts worth to get a solid 10w40. I assume that's what you want.

Here is the vi results at 40c and 100c of mixing 50/50 Torco 5w-20 and Phillips 15w-40

71.2 @ 40c - 10.9 at 100c Brew

The Torco was 40.88 @ 40c - 7.4 @ 100c

The Phillips was 115 @ 40c - 15.2 @ 100c

[ April 01, 2004, 08:04 AM: Message edited by: Motorbike ]

Hmmmm, the 40C viscosity is about 9% below the arithmetic mean, but the 100C is only about 3.5% lower. I suspect the 40C is more reflective of the base oil and the 100C result is more a function of averaging the viscosity index improvers (VIIs) together and then working them off the new base oil viscosity. In other words, the presence of VIIs is not accounted for in a basic viscosity mixing equation because the 100C viscosity is not a natural viscosity but artifically induced by the VIIs.

Sounds good. The question now is, am I right?

Pedro; Thanks for covering my six.
I hate when I've got goodies to deliver and my computer
won't cooperate.

No problem. I had the Viscosity Handbook in PDF and found the chart on the last page.

Pedro

Maybe I'm missing the obvious..I have seen that chart before. If you mix 20% 90cSt oil and 80% 10cSt. oil-all I see is 2 points on a vertical line. So whats the answer here and how did you get it from the chart

quote:

Originally posted by Al:
Maybe I'm missing the obvious..I have seen that chart before. If you mix 20% 90cSt oil and 80% 10cSt. oil-all I see is 2 points on a vertical line. So whats the answer here and how did you get it from the chart

Wait-I see it..You first need to make a line goint from 100% 90 cSt. and connect it to 100% 10cSt, and then pick out the percentage you have. duhhhh

That chart only works for straight viscosity oils...no viscosity index improvers.

For common multi-grade oils, i. e., 5W-30, 20W-50, etc., only a lab viscosity test will give the resulting viscosity of the mixture.

Ken

dos the equation really work?

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