generally anything that is a surface of contact for moving parts, anything that acts as a seal for gas, has to be lubed. You can pretty much see the wearing on the parts of your gun by opening it up. Don't forget to maintain your mags as well, which is the same process but you're unlikely going to open up your gun unless there's debris stuck in it. I usually empty the gas, one drop on the output valve, give it a little "massage", one drop on the fill valve and fill the mag just a little bit to put pressure inside the mag.
Here's a good guide:
https://www.youtube.com/watch?v=KYGqq0d7yzM