This is 1602 OLED, using WS0010 controller, and besides the standard mode, it also has Graphical mode. So you can write whatever you want. Or you can use a trick - Draw something with these 8 custom chars. Switch display to graphic mode - all image on screen remains, but it is turned into graphics. Now turn back to standard mode, update custom charset and draw next symbol at next place (the symbol you've drawn previously is kept on display, because it turned into graphics) and so on.