Two possibilities,
Dirt/dust in the lock is mixed with the lube and causing it to gel and
bind when it's colder.
Because the lock wasn't used for a long time it has a small amount of
corrosion in between the cylinder and the plug (rotating part the key turns)
Either way -
Go get a can of brake cleaner, make sure it has the straw.
A can of compressed air - with a straw (or if you have a compressor a
small tipped blow gun)
and some graphite based lock lube.
Now use a bit of tape and tin foil to mask around and under the lock
(mainly to keep you from spraying the cleaner around and to protect the
paint during the next step).
Hold open the key door, stick the straw in as far as it will go.
Give it one short blast of cleaner.
Now insert your key and try to turn it. It probably won't turn very well
if at all.
Repeat the cleaner and key until the lock frees up. The cleaner will
spray through the tumblers and wash out the crud around the plug,
turning the key acts like a scraper and will break loose the crud so the
spray can work.
Now once you get it freed up. Hold the key door open and use the air to
blow through the lock and dry it.
Apply the lock lube into the plug and apply a quick shot of air to push
the lube out around the plug. Apply some more lube and repeat until the
lock works the way you want it to.
Don't use any type of liquid lube in the lock, it just gums them up and
causes problems.
--
Steve W.