Arduino at 5V but 8MHz
Only difference between the regular Arduino 16mhz bootloader and the 8mhz bootloader is that the 16mhz requires an external 16mhz crystal. The 8mhz bootloader uses the internal clock source instead. Less accurate.

AND the lower the voltage, the lower the internal crystal can work at. But in your case, there is no problem with running at the internal 8mhz at 5v.