Older engines may have problems with rubber parts dissolving.
High mileage engines may start using oil because accumulated varnish deposits that are taking up ware may wash away.
All engines will have worse fuel economy because the fuel has less energy per gallon.
Engines with 'closed loop' fuel injection will have even worse economy because any 'oxygenated' fuel will fool the injection system into thinking the mixture is leaner than it really is. This can usually be cured by re-calibration.
