I used a kill coil. Way less fuss than having to worry about oil or drops of whatever. Just stick it in the reservoir and call it a day.
Those coils are only for preventing biological growth, like algae, in the loop. It doesn't prevent corrosion. If you don't have differing metals in your loop, you shouldn't have to worry too much about corrosion.
I only have a tiny bit of ethylene gycol in my loop to lower viscosity of the water. (Helps purge out air bubbles.)
Very true. The solution that comes with the kit has the anti-corrosion in it. If you use distilled water then yes you will need something for that.