mixing oil weights.

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Mar 26, 2009
Is there a conversion or calculator I can use that would tell me what weight oil I would be using if I mixed weights?

Ie. 5w20 with 5w30 and 0w30? or maybe 1 qt of 0w40 to 4 qts of 30 wt.
What is the point? Any Frankenblend is just a guess unless it is equal parts of say 5w20 & 5w30 = 5w25. But mixing a straight weight with a multigrade...? Beats me...
I think you're better off looking at the hot cSt of what you're starting with. Example 5w20 may have a cSt of 8.3, Ur 5w30 might be Cst 10.5, so equal parts give a cSt half way in between
(8.5 + 10.5)/2 = 9.5 then if U check the definition of oil by viscosity, U will see that 9.5 puts it into the VERY thin 30 weight range. 30 weight goes from about 9.3 to 11.4 100 degrees cSt, so you can call what you made 5w25, but more important is what is the hot and cold viscosity, and where Ur pour point ends up, U brewed an oil of cSt 9.3, and then use your judgement from there. For myself, I use this particular mix even for engines that call for 5w30, and they run just fine, and it should work fine also in engines that call for 5w20.
Wasn't there the Shell calculator that some people used to figure out their homemade oil blend ?

I'm using a 50/50 mix of 5w20 Pennzoil dino and 10w30 Havoline HM in my wife's Accord.
What happens to the pour point? Even though you're mixing a 5W20 with a 5W30 50/50, the 5W20 could have a different pour point, than the 5W30. I'm wondering what exactly the outcome will be, do we average the pour points as well?
well, I was just thinking. I know some people mix a mobil 1 15w50 to there regular weight oil just to boost the visc some. I have heard that mobil 1 0w40 was a good oil.

Hypothetically speaking of course, lets say a car calls for 5w20. And someone replaced 1 qt of a 4.5 qt capacity with 0w40. Would this cause any real harm?
Case in point. I just think it would be better to replace a qt with a good synthetic motor oil, then using any oil additives like a lucas or stp.

Now valvoline synpower oil treatment would be a different story. I wish they still made that.
With any multi-viscosity oils you never know exactly what you'll end up with unless you test.

Who cares. Just estimate and drive away happy.
I usually type in the numbers to a spreadsheet, get a total on a column and then divide by the numbers of quarts.

10 40
10 40
10 40
20 50
5 20
5 20
60 210

divide by 6 on each column


Of course, as others have mentioned above, it is only a calculated guess, as only oil analysis can give you real numbers.

The example above is exacly what I used for the recent fill on our Jeep Wrangler (89 with 170k miles), as I had a mixed up stash of oil I needed to get rid of. That oil will stay in the Jeep for 3 months and then it will get a fresh pour.
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the 10, 20, 30 40 numbers are NOT viscosity or even meaningful numbers other than to indicate the RANGE of viscosities you are in.

IF (a big if) there were any benefit in mixing grades of oil you would be better off using the volume-weighted average CSt viscosity values.

I don't understand the idea of mixing viscosities - your engine will never know. Just use a weight of oil which has the target viscosities you are tyring to achieve and be certain what you are getting.

If you are on 5w-20 and you add in say a quart of 50 weight then you may end up with what, a 30 weight? so why not use a 30 weight to begin with?

what benefit is there to this mixing? other than to feel in control?
Yeah, 10W35 has no meaning, it does not exist. What you can do is adjust the cSt to where you want it. I have also done it to reduce the cost of oil. I had a gallon of 10W30 Rotella T in the garage and 2.5qts of some Rotella T Synthetic I got for free. So I mixed the RTS with half a gallon of the 10W30. This is probably a 10W40. I know for a fact it is still a 40, the W part is a little questionable, but I figured a 10W mixed with a 5W should at least be a 10W.
I mixed it up for the mower.
Any of you back yard blenders mix different brands. EG 4 qts YB 5W30 with 1 qt of Mobil 1 15w-50? I agree that mixing a good synthetic oil into a dino oil is better than adding Lucas or STP. I plan on mixing PP with YB this fall for my Aerostar.
I'm about to do a super frankenblend in my moms car to use up my leftovers. 1 quart Mobil 1 10W-30EP, 1 quart Mobil 1 10W-30, 1 quart Valvoline Synpower 5W-30, 1 quart Super Tech 5W-30 synthetic, 1 quart Quaker State Torque Power 5W-30 synthetic, and 1 quart of Motorcraft 5W-20 synthetic blend.

I'll let you know if the world ends, or if it creates a microsingularity.
Sounds interesting. Next OC I have some Formual Shell 5W20 that I plan on mixing with Havoline 5W30. I don't anticipate any increased Global Warming.
Originally Posted By: demarpaint
Any of you back yard blenders mix different brands. EG 4 qts YB 5W30 with 1 qt of Mobil 1 15w-50? I agree that mixing a good synthetic oil into a dino oil is better than adding Lucas or STP. I plan on mixing PP with YB this fall for my Aerostar.

I have a 3 gallon Dispos-Oil container where I put my oil leftovers. The M1 0w-20 / 0w-30 mix I'm running right now came from there, but now I've dumped the rest of my opened oil into it. It's a mix of PP 5w-20, Syntec 5w-20, M1 5w-20, Pennz YB 5w-20, SynPower 5w-30, M1 0w-30, GC 0w-30, Chevron Supreme 20w-50 and Formula Shell 20w-50. It's got "dealership documented servicing" written all over it.

I'm thinking of measuring out 4 quarts into a PP 5w-20 jug and taking it to the dealership when my Civic's A1 service comes due in a couple of months. If I do the easy stuff myself, it'll run about $18 + an oil filter to have them change the oil and inspect my car looking for something to sell me. I'll probably leave the oil in there for the complete OCI if the engine feels fine.
Well, at least I'm not the only one that thinks about this stuff. I guess we all mix oils as you never get all the oil out of an engine from the last change.

I have mixed mobil 1, valvoline synpower and castrol 0w30.

For that matter, the mobil 1 was a 0w40 oil. I think that is the only mobil oil I would use at this point
My frankenblend went in today. The engine in my moms car seemed smoother and quieter with it. Though, the previous OCI was a 3,665 mile Auto RX rinse phase with Castrol GTX 5W-30, so I'm not really surprised that getting that dirty stuff out and putting a mostly synthetic mix in made the engine sound better.
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