Locks sticking with keyed entry (ford ranger)

A friend is having issues with his Ford Rangers locks, and he feels if he turns them any harder he will destroy the locks and not be able to get in. The truck is only 10 years old and in great condition, for a ford. He says it only happens when it's cold outside, but it's delaware, no snow or rain conditions. Is there something in the lock that can freeze at low temps? He is not sure if he wants to try a lock de-icer. My idea is to dip the key in some oil, stick it in and just turn it a few times to loosen it up...But I don't want to give bad advice. What's the best approach here?

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I am having the same issue with my 2011 right now and I live in texas lol( around 20 degrees w/o windchill)
I decided to try my passenger door when it does it and it's a lot easier, so I was thinking about popping off the door panel and removing the mechanism and squirting it with a dry lubricant.

 

Go to the lock section of the hardware store and get some lock graphite and follow the directions squirting it in there.

Makes a big difference.