Oil math- does blending make a mean, or a mix?

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Application is '06 Saab 9.3 SS/2.0T (turbocharged)- for which i'm contemplating tailor making my own blends of weight/brands of oil depending on season/operating conditions/etc. If nothing else, consider it a thought exercise/conjecture lol. so i'm curious...
When you blend 2 (or more) weights of oil, does it end up as the mean/average number? or does it just make a mixture, with the two weights remaining distinct?

For example-
1 part 0w-20 + 1 part 10w-40= 2 parts 5w-30? Or does it equal 2 parts of a 0w20/10w40 mixture?

If it is the latter, it would be UNdesirable, because (in summer for instance) in the hot turbo engine the parts of 0w20 could brake and/or fail early, correct?
 
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I really like your thought process on the latter, but it doesn't work like that (thankfully)...all the stuff in the bottles will be quickly mixed.

While the component parts, in and of themselves may be more or less deteriorated depending on what they are (not specifically related to 0W20 or 10W40), they will have half as many trips to the turbo, top rings, or whatever.

As to the former, roughly speaking, the results will be close.

You need to use a mixing calculator for the (likely) viscosities, and do some fancy maths for the (likely) "W" grade that it could end up...the results of mixing finished multigrade engine oils are not necessarily predictable as some on here would posit.
 
If you use the actual viscosity numbers (cSt), and weight your math with their ratios, you'll get pretty close to what your finished blend will be. To my understanding, the viscosity scale is linear, making a direct comparison like this possible. But you have to use the actual viscosity measurements -- not the grade (0W-20, etc).

If you used 4 quarts of a 20 grade (with a viscosity @ 40C of 8 cSt) and 1 quart of a 40 grade (with a viscosity @ 40C of 14 cSt), your finished blend would have a viscosity @ 40C of about ((4*8)+(1*14))/5, or 9.2 cSt. Change it to half and half, 2.5 quarts of each, and your viscosity @ 40C would be about ((2.5*8)+(2.5*14))/5, or 11 cSt.
 
Originally Posted By: Hokiefyd
If you use the actual viscosity numbers (cSt), and weight your math with their ratios, you'll get pretty close to what your finished blend will be. To my understanding, the viscosity scale is linear, making a direct comparison like this possible. But you have to use the actual viscosity measurements -- not the grade (0W-20, etc).

If you used 4 quarts of a 20 grade (with a viscosity @ 40C of 8 cSt) and 1 quart of a 40 grade (with a viscosity @ 40C of 14 cSt), your finished blend would have a viscosity @ 40C of about ((4*8)+(1*14))/5, or 9.2 cSt. Change it to half and half, 2.5 quarts of each, and your viscosity @ 40C would be about ((2.5*8)+(2.5*14))/5, or 11 cSt.


My understanding (No, that's wrong, start again).

My Googling suggests that with straight grades its calculable but isn't an arithmetic mean as above, but is skewed towards the lower viscosity component. With multigrades it has to be determined experimentally.
 
I wouldn't have to worry about blending a final quart if jugs came in 6 quart rather than 5 quart sizes. To those with 5 quart engines, there's little reason to blend.....except to get rid of all those cheap AAP/Auto Zone 5c to $2 quarts you stashed away each December.
 
Since oil viscosity is a range its more complicated.

you could add 2 quarts of 10w40 to 3qt 5w30
and it could be a 5w30
a 10w30 or even a 10w40

And if they are different brands the different additive packs may not work well together.
 
I happened upon a bunch of really cheap Mobil 1 15W50 several years ago. I've been mixing it with 0W30 "magic green" Castrol and it seems to work just fine in my GM 3.6 V6.

7.5W40 or whatever, LOL
 
Originally Posted By: nascarnation
I happened upon a bunch of really cheap Mobil 1 15W50 several years ago. I've been mixing it with 0W30 "magic green" Castrol and it seems to work just fine in my GM 3.6 V6.

7.5W40 or whatever, LOL


Since of course that grade doesn't exist, odds are it is a 10w-40, maybe a 5w-40.
 
Originally Posted By: Ducked


...My Googling suggests that with straight grades its calculable but isn't an arithmetic mean as above, but is skewed towards the lower viscosity component. With multigrades it has to be determined experimentally.


Agreed, as that will be the case.
 
Thank you all for the thoughtful replies and advice!
smile.gif


i guess the main aspect i was curious about was, making sure the 20 part of the 0w-20 wouldn't remain that low and get cooked early in the turbo and prematurely clog the oil filter.

If my thought pattern seems a tick off, i blame it on the lack of sunshine during the winter solstice, LOL
 
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