If that's an '86 4Runner, then I suspect that it has a few miles on it. If that's so, then you are probably getting some blow-by with the lightweight M1, whereas the heavier 5W40 is giving you better compression and therefore better fuel economy.
I've been running Shell Rotella T synthetic 5w40 for a number of months in my '86 4runner and have been getting a consistent 25mpg (2.5% less corrected for tire size/wear, so 24.4mpg). A few weeks ago, I switched to M1 10w30. Fuel economy dropped 5% to 23.8mpg, at least over the first few tanks. I'm using the exact same gas (Chevron 91, additive was switched from ethanol to mtbe in sept, but I was still running rotella then and was still getting 25mpg), driving the exact same routes to work, temp's are pretty much the same, a tad cooler (temps around 80/50F rather than 85-90/60F, So. AZ). Nothing else had been changed. The only explanation I can come up with is M1 10w30 has more friction than Shell Rotella T synthetic 5w40. BTW, with M1 Delvac 5w40, I was getting about 24.1-24.5 mpg, so the Rotella is still the mileage champ.
I just started useing Rotella T 5w40 in my Wife's 2003 VW Golf 2.0 about 4 months ago. Initially, I used Mobil1 5w30 immediately after break-in. We decided to switch over to Rotella strictly based on lowering maintanence cost since I change it every 3-5k miles. (...also because 5w40 is the recommended weight per VWoA) We've noticed no change in MPGs with the switch, just a slightly fatter wallet.
It's not Rotella in particular, it's the thicker oil. I have seen the same thing when I switched from 10W-30 to 10W-40 in my SAAB and so have other people. Try any 10W-40 or 5W-40 oil and you should see same fuel economy numbers as with Rotella.
The theory I have is that the thicker oil reduces internal friction more than energy wasted due to thicker oil. This seems to be the norm for engines with 70k-80k miles or more.
Slalom44's answer does makes sense. I did install new valve seals, HG, exhaust valve guides, TRD cam w/ new rockers 2 yrs ago when I did the t-chain, but everything in the lower block is original. Oil burn off did drop from 1 qt every 3K mi to 1/2 qt every 3K mi after the valve work. With the 5w40, it's at most 1/2 qt, if less. In my '01 honda prelude SH, oil consumption (from new to now, which has 16K mi) was 1 qt every 3K mi on dino 5w30. After switching to M1 Delvac 5w40, consumption was essentially zero (like 1/4 qt). That issue was obviously the thicker oil, as the 5w30 dino was like water.