I like Rotella T syn 5w40, too. I've run Delvac 1 5w40 (liked that, but pricey), then tried Rotella T syn 5w40, then tried M1 10w40, will switch back to Rotella T syn on the next change. My fuel economy was best w/ the Rotella T syn (this is for a '86 4runner w/ 4-cyl, fuel injected, 157K mi engine). With the Delvac 1, I got about 24.1-24.5 mpg. With the M1 10w30, it dropped to 23.5-23.8mpg, while w/ Rotella T syn 5w40, it averaged a consistent 25mpg (these are averages over 3K mi). These mileages are to/from work (22 mi each way, maybe a dozen traffic lights, surface streets, light foot). BTW, I usually run Chevron 91 w/ techron fuel. The best I ever got was 27mpg down from Flagstaff to Tucson (loosely drafted a big-rig for the last 100 miles--just far back enough so I could see the driver's side mirror). That was running Shell 91 gas w/ Rotella T syn 5w40.
I read specs showing the TBN (total base number) on M1 10w30 is only 6, while w/ Delvac 1, it's 12 and with Rotella T syn, it's 10. This suggests to me that extended intervals using non-0w40 M1 is more marketing hype. Looking at the specs, the chevron synthetic (I think 5w40) seemed the best of any brand, but I've yet to see that anywhere.
I use the rotella T 15w40 dino on the stinky as its got the "C" class for diesels
I never looked at the Syn stuff to see if it also has the the "S" and the "C" class on the SAE circle
If so then I may have to start getting it by the drum