0xfd instead of 0x05 -> 0b11111101 and 0b00000101
Extended fuse (efuse) only uses the 3 right bits, the 5 on the left are unused and often read back as 1s. The warning can be ignored, or you can change the boards.txt setting from 0x05 to 0xfd to make it go away.
If you want to run with no crystal, just the internal RC oscillator, you need to make the low fuse byte have 0010 in its 4-right hand bits.
"Calibrated Internal RC Oscillator 0010"
If the chip was already set up as an Uno, it needs to have a crystal connected to be able to change the fuses to run without a crystal.
Robin2,
I always use the internal 8MHz oscillator as that minimizes the number of external parts.
What board type do you select for that? Or, do you have a custom board set up in boards.txt that you can share?