It is best not to alternate repeatedly between gasoline and E85. If you do switch fuels, it is recommended that you add as much fuel as possible - do not add less than 11 L (3 gal) when refueling. You should drive the vehicle immediately after refueling for at lease 11 km (7 mi) to allow the vehicle to adapt to the change in ethanol concentration. This is per the gsi.xw.gm.com SI Document 2344147.
Source: gsi.xw.gm.com new SI document 2344147