160° f/71° c is the safe temperature. It's up to you and your tastebuds if you want to strain it. Straining it of course will stop the infusion so if you don't like it that spicy I would strain it but if you do then leave that in there. Make sure that you refrigerate your oil in an airtight container after it's cooled down. 20 minutes at 160 ° f is a perfect amount of time in my opinion. Technically speaking it could be as cool as 140° f or 62° c. I don't want to affect the herbs with heat that is too high and could draw out some bitter flavors that I'm not wanting. This is especially true when infusing oil with garlic. One exception to the rule. If you're making garlic oil it should be heated to 180° f to kill any possible pathogens. Low and slow is really the name of the game. We're not looking to cook the oil but instead just a low simmer.

Use an oil with a high smoke point such as avocado, peanut or safflower. People also say that adding oil to butter will increase its smoke point but that is incorrect. The milk solids will still burn even if the oil doesn't. I'm not really sure what your goal is with adding that cold oil. Adding cold oil would only stop the cooking process. Preventing burning is done with temperature control and not by adding cold oil which would just stop the infusion process and not prevent burning. A probe thermometer is necessary to verify temperature.