C5ROADRUNNER

Checked the oil today (cold engine) on my C6 and was just under the line so I added a half quart or less of 5W30 from the last change and it just barely went over the low fill line. I have a couple of 10W30 quarts left over from my C5, is it taboo to mix the different weights?

need-for-speed

I don't see any harm in topping it off w/ 10W-30. You'll have 6W-30

Are they the same brand ?

C6Matt

Do not mix diffrent weights of oil...Im an engineer in GM powertrain, so I *might* know what Im taking about..Diffrent brands of the same weight is okay though ...

C5ROADRUNNER

Thanks for the tip, looks like I need to go to Walmart for a refill.

need-for-speed

O.K. nevermind. I defer to Matt.

arcticblast

Thanks, I didn't know that.

LTC Z06

Why?

C6Matt

That is a longer answer than you might think but basicly multi viscoity oils do not blend well... the long answer and effects I do not have the time to type...

LTC Z06

I'm sure it is a long answer, by not blending well this causes the oil to not circulate well, or stay between the bearings properly?

arcticblast

On the back of a quart of M1 5w30, it states "compatible with conventional oils", but no mention of mixing viscosities. Glad there's an auto engineer to enlighten us. PS- I've got a petroleum engineering degree, but it's been a few years. I've slept since then.

C8-Vette

need-for-speed

Hey! Me too !! But since I graduated in 1986 (no Pet. jobs to be had) I had to go to work for a "Yankee Pipeline" Co.

2005ArcticWhite

badco

You sure you don't have that backwards? You'd recommend mixing Pennzoil and Castrol, but you wouldn't recommend mixing 10w30 M1 with 5w30 M1?

Curtis A. Franz

On the other side of this coin is this chemist who had all kinds of connections to Amoco and has a Ph. D. The viscosity weight labels are ranges of viscosity and additives don't really care about their fellow components especially if they are from the same company. In other words, I don't buy that one shouldn't mix. Lets get specific here if you are going to make those claims.

SimpsonH

So, you are saying that there is a problem mixing slightly different multi-viscosity oils from the same manufacturer/refiner (same type oils with similar additive packages), other than changing the viscosity of the oil based on the mix ratio? For example, a quart of Mobil 1 SuperSyn 10-30 will not blend correctly with Mobil 1 SuperSyn 5-30 and you should run the engine low on oil rather than mix the two if this is your only option? Can you explain the problem this creates and the damage that will occur?

I think many of us would like to know the long answer because when you add a new quart of Mobil 1 SuperSyn 5-30 to Mobil 1 SuperSyn 5-30 that has been used for 3000 miles you are actually mixing “different weight” multi-viscosity oils with slightly different additive packages (oils shear and loose weight during use and the additive package ages).

Quoting Stephen Miller from Shell: “For instance, if you mix viscosity grades such as a 5W30 low-viscosity oil and a 10W40 higher-viscosity oil, it is reasonable to expect that the resulting product will have viscosity characteristics which are thicker than the 5W30, but thinner than the 10W40. This change does not reflect incompatibility - it's simply a re-balancing of the viscosity characteristics. In all other ways, the product will work as expected. Using our Shell products as an example, if a car specifies a 5W30 oil as the preferred viscosity grade, then FormulaShell 100% Synthetic 5W30 provides the best performance we have, Miller says. And if you find you need to top up with another Formula Shell there will be no compatibility issues, but the mixed product's performance characteristics will not be the same as if only 5W30 FormulaShell had been used (it will have blended properties which is not necessarily bad since it can improve the viscosity of aging oil). But there's absolutely no danger about incompatibility resulting from mixing Shell engine oils. We've tested all of our grades and brands, and we have never observed a problem."

You will find similar statements from all of the major companies.

famous1

As long as you do not need the outside range of either weight, you will be fine. It will only be diluted, no significant measurable problem will occur. IMHO

VET4LES

benspruce

At my first oil change I switched to Mobil 1 10w/30. This was at the recommendation of a corvette tech (not at the dealer). He said the 5w was a little more than water and that GM was recommending 5w for reasons not related to engine performance. Anyone care to comment? Matt?

TheGreatOz

The reasons are...?