unleaded 88 octane vs 87 octane... an informative article on the difference

the major difference between Unleaded 88 and Unleaded 87. Unleaded 88, also called E15, contains 15% ethanol and 85% gasoline, whereas the Unleaded 87 you are familiar with is a blend of 10% ethanol and 90% gasoline. 

that increase in ethanol content may result in a reduction in your MPGs. It's simply because gasoline is more energy-dense than ethanol, so the lower gasoline content can result in lower fuel economy. 

 pre-2001 vehicles should not use Unleaded 88 or E15 gas is that their fuel systems are not designed to handle the higher alcohol content of these fuels. Some issues related to the use of higher-ethanol-content fuel in vehicles not designed for it include clogged fuel filters, fuel tank corrosion, swelling or cracking of plastic fuel system parts, difficult starting, and failed fuel hoses.

https://www.slashgear.com/2155211/87-vs-88-octane-fuel-grade-differences/