I just used motor oil on mine. No problems after ~30k miles
+1 for motor oil, but I've also heard of people using straight gasoline, too. Not sure why?
The Toyota factory service manual recommends using gasoline as the lubricant for the o-rings.
I think silicone grease is the way to go, since that's what MotorWest Performance sends with their injectors and they're experts in the field of injectors.