How to properly use the 4N25 Optocoupler
"Can I use an arduino to control 12V 70mA load?"

No, not without some additional circuitry around the opto.

With a CTR of 20%, and needing to sink 70 mA on the output transistor, you would have to pump 350 mA through the input diode.  The absolute maximum forward current for the 4N25 is 60 mA.  No can do.