Unless it's been in the kitchen or hanging over a smoldering candle or by a camp fire I can't imagine that it needs cleaning. I hope you gently dust it regularly. And, if you really think it's dirty....dip a cotton ball in a solution of gentle dish soap like Ivory....wring it out very well and just dab at the painting. If you are getting off dirt....continue....but in today's houses which are not heated by wood stove and lit by candles and kerosene I sure can't think it would need any sort of professional cleaning.
Linda C

No, no, no. Don't use water. Use bread. Google "cleaning oil painting with bread". Better yet, ask your local art museum.

The amount of water in a well squeezed out cotton ball is no more than in a slice of bread.....and the cotton ball contains no fat.

Smokers make paintings a mess! It may well need cleaning! It's hard to get that film off.

It ain't just smokers and you better believe that other things cause oil/grime accumulation on paintings, some of which are actually caustic. If you have a gas fired furnace, efficient as they are anymore, or cook with gas or have gas fireplaces, it also leaves a film on surfaces. I used to live near a huge mine and cement facility. The fine dust was also abrasive and ruined paint and enamel finishes and was impossible to keep outside unless you lived with the doors and windows shut. One doesn't do that in the country.