I'm fairly skeptical of the little phase separation based testers. I'm not sure what their claimed margin of error is, but it is a very unscientific form of analysis. And that's a significant reported spread.
While blend isn't analyized in-house very frequently, we have a 30K dollar piece of equipment to analyze EToH blend...just to offer some perspective. And I believe it is infrared and doesn't rely on full phase separation to occur to test the sample.
Anyway, here's the deal on varying E blends...
The oil companies are mandated by the Renewable Energy Standard to distribute X amount of renewables every year, a value which is always increasing, year to year. So, something like 95 percent of all gasoline distributed in the US has been oxygenated with ethanol, nearly all of which is at 10 percent blend. And while blending methods differ, typically either ratio blending or splash blending, they're both very reliable and accurate. And generally speaking, it just doesn't leave the terminal with some wild spec blend. I'm an operator at one of these terminals. If blend is out of spec, due to a mechanical issue or driver error, I don't release that product to the streets. Any terminal that isn't some hillbilly operation operates by similar standards. And remember, there isn't a wide margin for error, becasue the oil companies WANT the renewables in their fuels, to satisfy the governemnt standards.
All of that said, that doesn't mean conventional gasoline, without feedstock blending doesn't exist, isn't distributed. However, nearly all large urban areas require RBOB or CBOB distribution exclusively, both of which are blended with EToH (Reformulated/Conventional Blendstock for Oxygenate Blending). It really is regional. In areas like Austin, Houston, DFW and such, you won't find this phenomenon, as conventional may not be distributed anywhere in the immediate area. And the risk of selling conventionalal in an RFG, VOC/RVP regulated area outways any cost savings from hauling in conventional from outside of town.
If you're finding erratic consistency in E percentage, it is purely at the fault of the retail station owner. He's taking in gas based on price, shopping the market with disregard to fuel type. So he may buy a truck load of E0 conventional or something and a week later buy the RFG E10 because it was cheaper.
There's only a handful of states which require labeling on gas dispensers. So most just toss the "Up to 10%" decals.
Maybe if you become friendly with the owner of the station, he will share details or BOL/manifests of his gas purchases.
