If the lock is just very stiff when you turn it probably the PO lubricated it with grease or oil that has gotten dirty and is stiffening up as the temp drops. Graphite is really the only thing to use in locks since it won't stiffen up. You may want to remove the tumbler (as Brian suggested),clean it and lube with some graphite. If your problem is the key "hangs up" and won't turn unless you wiggle it then it's your lock wearing out. If so you'll need to get a new tumbler from the dealer and replace it.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold