try a small solution of fabric softner and dawn soap, with a small brush. That is how I get the nicotine off the walls in smoking rooms at a motel I work at.
My cousin, who works in an art gallery, said to clean it with rubbing alcohol on a cotton ball. Don't soak it with the alcohol, just wet the cotton ball and rub, in small circles, moving along a little at a time. Don't scrub, just 'massage' it gently, and take your time. Replace the cotton ball as it gets soiled. This method works on, not only oil paintings, but acrylic, as well.
As an artist I would NOT recommend using alcohol to clean acrylic paintings...the alcohol is actually used to REMOVE acrylic paint when we make an error in our painting! ! It is great and harmless on oil paintings, however!