If they are sticky they aren't clean yet. Try ammonia and Ivory Liquid dish soap in warm water. Get some gloves because this concoction will snatch the oil from your skin in a hurry. Easy on the ammonia, ok?

If you're shy about ammonia and soap try the GOJO white paste. Rub it on with a damp cloth and rinse it well.

EDIT: It was wrong of me to suggest or accuse you of not cleaning the armrest properly. Other conditions may cause vinyl to become sticky. SEM 39644 tac free will be a good remedy in cases where plasticizers are the issue. Pardon the quick response, please Sir.

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84 300SD
85 380SE
83 528e
95 318ic