I hear a lot of people talking about how their MacBook is too hot. They look at on-line forums and see that their MacBook is running at 83 C and get very concerned. They learn from the forums that they need to re-apply the thermal paste to keep their computer cool, but virtually all the forums participants are amateur idiots that offer ridiculous procedures that are likely to damage your computer.
This article is the right way to address this problem.
First, let's talk about why your computer is running so hot.
Computer CPUs get hot. They're designed to get hot, consuming many watts in a very small surface area. Ever touch a 25 watt light bulb that's been on for 5 minutes? It will be painfully hot. Take that 25 watts and put it in a much smaller package (think halogen), and it will be MUCH hotter. That's a CPU - a 25 watt device in a very small package.
CPU manufacturers put temperature probes inside a CPU not so that you can look at the temperature, but so that the computer can regulate its own heat. No heat sink is going to prevent that CPU from getting hot. Instead, a heat sink is designed to take that heat and spread it across a larger surface. Now if that heat sink was huge, you'd be done. But a laptop is small, so a fan is then used to blow room temperature air over the heat sink.
Your laptop has fan circuitry that is designed to regulate its temperature. The fan's cooling ability is based on the ambient air temperature and airflow. If the computer is feeling hot, it will spin up the fans to exhaust heat. The amount of cooling is based on airflow and the temperature of the air going into the machine.
So, what about thermal paste?
Thermal paste is simply some heat conducting goop that helps the heat to move from the CPU to the heat sink. Applying thermal paste is as simple as scribbling with crayons. And it will last far longer than the life of any machine. And therefore, "poorly applied thermal paste" or "worn out thermal paste" simply doesn't happen.
All that said, computers can get too hot, particularly when they get old. Why? It is almost always due to dust clogging up the airways of your computer, preventing the fans from blowing air over the heat sink. When a modern computer gets too hot, it will automatically power off. If you have that kind of behavior, the right response with your computer is to see if you need to de-dust the air passages. Another possibility is that the fan has failed.
So, in conclusion, leave the thermal paste ALONE. Ignore the forums, as they are filled with idiots and shills that market stupidly overpriced thermal paste. Let your Mac regulate its own temperature. If the fan is spinning too hard, it is because you have a runaway process, or because your computer is full of dust.