I've heard:
- Lye needs to be at (90, 100, 110) degrees
- Lye and Oil need to be within 10 degrees of each other
- Lye and oil should be under 100 degrees
The only hard and fast rule I've found is that the oil/fat and lye should be as cool as you can get them without seizing if you are using hard fats. I thought however if warm was good, hell why not just mix them both hot? I melted my oils (I use Stearic), fresh mixed my lye, and poured them together just KNOWING everyone else had silly rules. Well I got a volcano.
I found that my oil mix for my shaving soaps solidifies under 120 degrees so both the oil and the water are mixed over that temp. TOO far over and I get a volcano, too far under and the Stearic seizes.
For CP and oils, I have been doing completely room temp mixing and it's worked just fine. Take this for what it's worth from a newbie.