Optocouplers and power saving

Which Optocoupler would you take as input for maximum power saving.
Most off the time i use 4N35 but i wonder how low you really can drive
the LED and still are save.

Lets say i use a 20k pullup on the arduino pin thats 0.25mA.
So what resistor do i need in front of the LED?

The LED can work at a couple of mA. Just keep lowering the current until it does not work then raise it a little.

Weedpharma

It will switch somewhat slower at lower powers, be aware.

I found one that works with 0.5mA: 6N139