Viscosity blending–black magic? Destroying my M3?

[stock type sh]

The mix:
3.5qts 15w-50 Mobil 1 EP with 3.5qts Mobil dino 5000mi 10w-40

I’ve been blending for 11k miles now on my s52 powered 97’ BMW M3 with 93k on the clock with no reports of oil loss/consumption or potential engine damage. Before this, the changes were anywhere from 5-7k miles on anything from 10w-30 to 20w-50 or 15w-50 by the owners. The first two changes were M1 15w-50 and Valvoline high mileage 10w-40.

The reasoning behind the blend is a follows – These aren’t facts, just stuff I’ve been able to put together after reading the forums so use this guide loosely - I’m no chemist by any means, so I could be completely wrong about my assumptions below:

Daily drive 8mi round trip commute in stop/go Atlanta traffic – Synthetic protection wanted

93k mile old motor – let’s remove some buildup and clean passages – Extended
performance/5000mi includes detergents/cleaners package

Short oil change intervals – Mixing dino with Syn have an economic $ balance and the short changes are to flush out deposits/4000mi old oil.

I track my car monthly at Barber, Road Atlanta, etc..- Syn protection and high heat resistance wanted and a reason why 15w-50 was picked as ½ of the blend.

I like the 10w-40 for the “colder” mornings in the winter when it can get as low as 20 degrees (still “warm” for the Midwestern/northeastern people – I feel your pain – I’m from MN.)

I open to suggestions, bashing, ridicule, etc. I want to really know if I doing something that can cause a problem. I’m going to pull a sample for a UOA from this most current batch (Has a 1000mi round trip to Indianapolis, 1 weekend at road Atlanta, and 2600mi of daily driving on it)

Thanks for looking. Please share your opinion.

 

[surfstar]

I think that your UOA should answer your questions better than any of our guesses. Maybe invest in a Dyson analysis - that'd be my only positive input.

 

[Blue99]

Just my thoughts on your home brew -

Since Mobil Clean 5000 10W-40 is SM rated, quite possibly it contains the sodium based additive package. I think it's hard to predict the performance of the sodium add pack when blended with a more traditional formulation.

From a viscosity viewpoint, the viscosity improvers in the 10W-40 portion are being diluted by the 15W-50. The net effect is probably similar to a heavy 30 weight being blended with the M1 EP 15W-50.

I'm not anti-mixing & understand the appeal of a home-brew. But this one is a blend of dis-similar formulations to reach a viscosity goal that may be obtained with any good Euro style 5W-40.

But don't let my comments discourage your mixing endeavors!

 

[TallPaul]

Maybe you should run Redline 10w40 and go for longer change intervals (10 to 12 thousand miles if UOA supports). The Redline 10w40 will take the track in stride and that way you don't have to go so thick. Redline says you can run a grade thinner than with conventional oil.

 

[yannis]

I wouldn't experiment with blending in this joy machine.

 

[benjamming]

I believe surfstar is right on the money. We have a difficult enough time on here discussing "regular" oils much less home-made brews. That's not to take away from the fun of what we think will happen! In the end, a Dyson analysis & interpretation is the only way to go. I guess that might take a lot of the fun out of the "brewing"?

 

[beanoil]

I see no reason to stop your process. You wind up with a 16.2 cSt@100C and 118.7 cSt@40c.., in the range for a thick 10-40, you bring a good percentage of PAO to the party, and a I don't feel the sodium will hurt you in most anything but a GM 60degree V-6. Mix on, run it for as long as you see fit, and UOA it to watch for any engine issues.