No E15 for the Plug-In

Fortunately, the efficiency-drop isn't linear, as the energy-density tends to imply. It's actually that first 10% that causes the greatest hit. In fact, some vehicles barely differ going from E10 to E15 or E20.

As for what to do, I'll just use what's available. In 2 years, there's a mandate here in Minnesota to switch. They're won't be a choice. It will be interesting to see how things play out.

When E10 was mandated here year-round a decade ago and during the cold-season back in the 90's, there wasn't a backlash. So, I was rather amused when some states freaked out over the switch, claiming problems would occur... things that never actually materialized here.
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I think the most problems has been in small engines lawn mowers, weed whips these kind of things . The engines do not last as long, I think it's the heat thats hard on them.

 

Generally, you can use up to E50 before you have problems with the mismatch between the computer's fuel map and the energy density of the fuel. There are also some worries about increased corrosivity and problems with cold starts when the weather is cold. I believe Toyota has chosen an exceptionally conservative route by stating do not use E15, even though there is NO significant difference between E15 and E10. FYI, there are people on this board with Gen II and Gen III cars that run E50.

 

Are there any problems getting warranty service after using non-recommended fuel?

 

They would not be able to tell the difference between E10 and E15. Probably true for E20 as well.

If you ran E50, then you would have significantly positive fuel trims and it is possible that it would be noticed, but it depends on the reason for warranty work. Fuel trims are not normally checked unless there is a specific engine/ignition problem. If you ran E50 or higher and got lean burn DTCs, or had a cold start failure, then it is likely that they would not cover you under the 5/60 powertrain or under the Platinum warranty if applicable.