Noticed that opening the trunk with a key was getting harder. Today the key would not turn. Still opened with under dash release. So...

Used some Teflon bicycle chain lube on the key and inserted key. Did this a couple of times and the key started to turn in the lock. Kept doing this several times and, voila, now works like new.

 

Not really the key that you needed to lube but the key-hole mechanism itself. I would have open the trunk, remove the liners, lube they lock and also probably spray wd-40 on the keyhole and insert the key turning it a few times to loosen it up.

 

No doubt it was the mechanism and not the key. I wanted to use a non-stick lube, rather than wd-40, and this was the easiest, quickest way, about 60 seconds!