Chosen Solution

What type and size of screws hold the heatsink in place?

Apple uses a collection of Pentalobe and Torx screws. The best thing is to get this tool kit Pro Tech Toolkit it will have everything you need. If you are just interested on the discreet drivers at the bottom of base page MacBook Air 13” Retina Late 2018 Repair guides lists out what you’ll need. But before you proceed I would strongly recommend you get a goods baseline on what you are currently seeing for thermals. I sue this great app TG Pro getting the full version will offer logging which allow you to track things quite well. You should also clean out the dust and debris using a soft paint brush and a can of can’d air to blow way the loos stuff in short blasts before you go into refreshing the TIM. One of the biggest issues I see is people over do the paste! Take a piece of single ply tissue paper that’s the size of the black silicon chip now wet your fingers and make a tight wad out of it, even that is too big a pea sized blob you’ll need! Even still don’t exceed it in size. HEre’s the guide to follow MacBook Air 13” Retina Display Late 2018 Logic Board Replacement Lastly, you may want to monitor your memory and drive space as you maybe just exceeding what this system was designed to support.