lol. Honestly, if you don't know where to put it, I'm sure you don't even know if you need it or not. I intentionally didn't tell you the answer, because, well, it's pretty much stating the obvious and oil that's too cold can still be harmful to your motor(this is why many use thermostatic valves).
The answer is SOOOO easy, I'll even give you a hint; Where's the other heat exchanger at?
Another hint: it's not the heater core.