On my latest trek down to socal from San Jose, I had just down an oil change from castrol edge 5w30 (Walmart) and usually drive around on 87 to Mobil1 0w40 (Walmart euro formula) + 91 octane. I noticed my gas mileage went from 31.3 to 28.3.

At the same time I also put a fresh air filter. I set cruises control on the 5 fwy to 75 mph, tires set 38 psi.

Half way down I filled up with 87 instead of 91 and my consumption averaged 30.1.

Anyone else noticed similar conditions?

I believe a lot of small engine cars have switched to 5W-20 because it allows better fuel economy. The -40 is a heavier weight oil so it probably had some neg. impact on your fuel economy.

many of things could be at play, tail wind, drafting, throttle position.
now that theres a clean air filter, allows more air in, more air = more fuel
were your tires 38 psi before?
temperature differences between the runs?
technically 87 has more energy, since theres additives for 91 to allow to combustion to happen later (also you shouldn't run 87)

To me, the differences you're noting aren't significant enough to warrant any real concern or further investigation. There are so many variables that affect fuel mileage as Salty noted so I would just attribute your mileage change to minor differences in those things. I also have never run my car on anything less then 91 as the cost difference is so negligible.

Definitely not venturing down 5w20 that oil is too thin and will have deleterious affects on engine life for a short gain on mileage

Y not use 87? Fuel injected cars can compensate with ignition advancement and retardation. As long as the ignition timing cannot retard enough to resist detonation then you are okay.

The r53 motor isn't overly compressed and I was cruising at 3k @ 75 mph on cruise control.

Rarely if ever do I draft unless I know the other drivers. Since I was rolling solo, I didn't draft anyone or anything.

When I head down to buttonwillow in my s2000 cr I usually tuck behind my friends voltex wing s2000 and I can definitely feel the drafting effects

87 octane on a gen1 mini WILL RESULT IN BURNT valves....
When they get very hot...they get soft...and wear rapidly...
After a few months of "saving 10-15¢" a gallon, you will likely need a $2000 head job....
MOST CARS and the gen2 cars CAN HANDLE 87...but NOT THE GEN1 CARS.

How did you ascertain the fuel was the cause? Did you use a wideband and log AFR? At any point did you do a uoa?

Not to mention the detonation (knock and pinging) that is caused by low octane fuel where higher octane is required. Detonation can do horrible to engines.

And it also could be that a winter-blend of fuel was introduced in the area that you purchased fuel. Winter-blends typically cause lower MPG by as much as 2%.

If knock and ping count is not being registered, then what exactly is the problem?

Also my inquiry was based on back to back gas tanks.