Chosen Solution
It seems simply to be a high-quality electrical tape, but just to be sure, could somebody take a micrometer to it? I suspect that it’s 3M Super 33, which, for reference, is 0.18 mm thick.
It doesn’t matter what tape you use there. It’s just there to seal the gap between fan and heatsink. You could use anything that doesn’t melt at 80 degreee C.
