Mixing viscosities...

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Sep 20, 2003
AB, Canada
When you mix, for example a 5w30 with a 15w50, what is the actual end result? Will I get a true 7.5w40 at a 50/50 ratio? I ask only because in Western Canada (Calgary South) I am unable to find 0w40 M1. I can find DOZENS!! of liters of GC 0w30 however. I have seen numerous mention of ...add a little of this weight or that; DOES IT WORK ???? I ask because my 93 Integra with 360 000km likes a little thicker oil, but winter is coming.........!

One more thing...GC IS green!!!; kinda freaks me out; not sure why...oh well.
This topic has been on here several times, and on Edmunds as well. You will get answers going both ways...some say you will be eternally ****ed if you ever even think about it, some say as long as the same brand is used it is ok, some say ......
Personally I don't have a problem mixing oil wts, been doing it for years. As for what the final viscosity rating is you would have to do analysis to get the exact answer. I have been told by some of the oilco tech folks that it is not an exact mathematical weighted average but it is close enough. Keep in mind that even the same oil wt (eg 10w30,10w40,20w50) will exhibit a range of cold/hot actual viscosities between mfgrs at given reference temps. Pour/flash point/measured viscosity values can be different between mfgrs for the same marketed wt of oil in other words.
Some people are hung up on voiding warranties, other people are more concerned with taking vehicles through a longer lifetime,,,my vehicles haven't been in warranty for several years now, so I could care less about it, I care more about my vehicles "mature" years these days, under warranty I would just use what the mfgr says though. A new engine with under 36K miles is not the same as that same engine after 100-200k miles on it.
see ya
Mixing is relatively safe if you stick with the same brand, although it's mostly a concern with longer drain intervals (if you're doing 3000 mile oil changes, mixing with other brands is not going to harm anything in other words)

It's final viscosity depends on the reaction between the two oil's viscosity index improvers though. So you won't always get an exact average.
About a year or two ago I called Mobil 1 and asked them about mixing different viscosities. They said it was just fine and I could mix until my heart was content. But to just think of it as thining a thick oil or thickening a thin oil. I mixed M1 15W50 ans 5W30 in equal parts until I was able to buy 0W40 in my area. Seemed to work great for me.
If you need something thicker, try Mobil's fully synthetic Delvac 1 5W-40, I think most Canadian Wal-Marts stock it.

It's over in the diesel oil section but it's also rated for gasoline engine use.

I think using Delvac 1 5W-40 would be better than mixing.
Hi geeeman,

Actually, if the vis change was a straight average of the different vis ranges, then a 50/50 mix of 5W-30 & 15W-50 would yield a 10W-40. To get a 7.5W-40 you'd need to start with 0W-30 & 15W-50.

The EC product is a heavier 30wt than most others, & meets ACEA A3 specs. If you can easily find it in copious amounts, then I'd say that's your #1 choice.

I also agree with quadrun1, that D1 is another excellent choice for your situation. Mixing M1 with M1 isn't a problem, but I think it should only be considered if your desired results can't be met with other, readily-available options.
Another reason that mixing viscosities doesn't give you an average of the two is that each viscosity has a range it fits into. In other words, you could have a 10w30 oil with a viscosity of 12.4cst at 100c, and a 10w40 with a viscosity of 12.6cst, but mixing the two doesn't give you a 10w35 (esp. since there is no such viscosity) since those two oils would be essentially identical anyways!
Exxonmobil/ Esso, has a very good 0W40 XD-3.
Its priced very well from the bulk outlets AND a PAO for those stuck on the base group thingie.

[ October 09, 2003, 05:32 AM: Message edited by: userfriendly ]
I checked the Esso station near my work this morning to see if they had that 0w40 but they didn't. They only had two viscosities of Mobil 1 also, just the 5w30 and 10w30, and sell it for $8.99 per liter! Robbery!

Originally posted by Ken2:
Mixing oil viscosities is NOT the arithmetical average. Open the Lube Specs Handbook, go to viscosity, and look at the chart on page 9 http://www.ethyl.com/nav/default.asp?sec=products&page=la

And, this chart is only for single viscosity oils, not multi-vis. You need to do a viscosity test in a lab to find the actual result.

I've seen that chart-I have yet to figure it out. Could you give an example so I can see how it works


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