For the past 2 months, something like oil has appeared on my K8 keyboard. I didn't spill anything on the keyboard but the oil stains seem to be spreading from the inside. What should I do?

This was caused by the lubricant that was used to lube the switches to make them smoother by the factory. Sometimes when the temperature is getting hotter, the lubricant comes out a little, but it will not affect the normal function of the keyboard. You can clean it up and keep using the keyboard afterward.