I simply take half an onion and I rub it on my oil paintings. When it becomes dirty, I cut a slice off and continue until the whole painting is rubbed.
The colors become brighter and the onion becomes dirtier. With me it functions! If you or somebody else will use this traditional method, make a try before (on a not so valuable oil painting or on a small part of it).
I think there are also chemicals that function well, or even better, but they are more expensive.
