M1 15W-50 vs 15W-40 HDEO

[fdcg27]

Many of us who own older Euros have used 15W-40 HDEOs and have recommended them to others.
I used Castrol Tection Extra 15W-40 for the current period last year in my old e36, while this year I've been using M1 EP 15W-50 in it, mainly because I had some and wanted to use it up.
I expected to take some hit in fuel economy, since the M1 is a thicker oil by every measure than the Tection.
The M1 EP has HTHS of 4.6, while Castrol does not deign to give this measure that I could find. The M1 is thicker at both 40C and 100C, according to the two PDSs.
In the nine hundred miles I've put on the car since last Tuesday, though, fuel economy appears to be slightly better.
I've actually averaged a little over 30 mpg, while I never bettered 29 mpg on the Tection.
My driving has been quite similar, including the same weekend trip on each oil on the same weekend in each year.
How could going up a grade not only not hurt fuel economy, but maybe even improve it?
The only thing that I can figure is that the M1 has a larger content of friction modifiers than the Tection.
The moral may be that you loose in fuel economy what you save in oil cost with a HDEO.
What experinces have others had with thick PCMOs vs HDEOs?

 

[dparm]

The HDEOs might have slightly higher starting TBNs since they're targeting diesels.

 

[Gene K]

Most of the 15W40 HDEO are 4.2 or 4.3 HTHS. I cant really explain other than I know esters can reduce friction and maybe the M1 has some.

 

[fdcg27]

Per Castrol, Tection Extra 15W-40 CJ4/SM, which I used, has a starting TBN of 10, while I can't find a reference to the starting TBN of the M1 EP, so I don't really know.
I do know that XOM claimed 15K in normal service for the oil, so I assume it had a pretty high starting TBN, and it does have a larger dose of calcium (a base) than does the Tection.
Still, I'm not sure what impact starting TBN would have on fuel economy.

 

[fdcg27]

Originally Posted By: Gene K
Most of the 15W40 HDEO are 4.2 or 4.3 HTHS. I cant really explain other than I know esters can reduce friction and maybe the M1 has some.


There was speculation over the years on this site that some M1 flavors, including 15W-50, had some Grp V content, but who knows?
I do know that I am happy with the oil in the car, and if the fuel economy improvement is real, the 4X as costly M1 will prove to be less costly over a 5K OCI than the Tection.
I bought both oils on clearance, and paid $3.74/gal for the Tection, while the M1 ran $3.49/qt.

 

[pavelow]

I'd run the M1 15W50 over the Diesel 15W40 if it were mine. M1's 15W50 is a great oil designed for hard performance car usage. Their OW50 is even better if you want a better flowing oil at cold start.