I have been running Unleaded 88/E15 in my 2015 6.4l and 2015 5.7 RAM 2500 and 2015 Jeep Cherokee 3.2l for many years now.
The 6.4l has had it exclusively for the last 3 years and about 75k miles. I think one time I couldn't get it and one fill up of 90 non ethanol for a dismal test.
Minimum of 87 octane and maximum of 15% ethanol is what the manual says.
It is safe to use in all vehicles from 2003 according to the tag on the pump. Since my 99 V10 is the same engine as used in 03, if i am going to use the amount of 88 pumped I will use it in it too. But, it doesn't get drove much, so it usually sits with 90 non ethanol in the tank.
I have found the 88 gets better mileage and performance than 87. I rarely use 89, since all 3 of my vehicles call for 87, but I have noticed no difference from 89 and 88 when I have.
The real deal is that here in Central PA it is 30 cents cheaper than 87, and 50 to 70 cents cheaper than 89. And Sheetz runs it at 2.99 often and sometimes 1.99 for promotion...so, regardless...I use a lot of gas and saved a ton of money.
176k on the 6.4l 135k on the 5.7l and 78k on the Jeep.