Before putting the key in give the cap a couple of hard whacks with the side of a clenched fist.
Then press down HARD on the cap whilst turning the key. That removes any load on the latch and can help.
Once you get it open, remove the security bolt and then strip, clean and lube the latch mechanism.
Putting lube down the key slot is mostly useless. You need to lube the latch mechanism and if it's corroded, strip and clean it first.
When you take it apart, beware that there is a little spring in there that goes "boing" and hides in the most inconvenient place in your garage. <LOL>
If it really is totally seized, you will need to drill the cap to get to the security bolt to remove it.
You will need a new cap after that.
This should help you figure out where to drill. It's the smaller of the 2 holes (the bigger is for the famous whistling vent).
https://www.ebay.co.uk/itm/146142409004