Depending on the type of grunge, oil, rust, or a combination of both, I'll clean them in Simple Green, Evapo-Rust or Hoppe's #9 gun solvent. Hoppe's works great for old varnished oil and grunge.
After they are clean I just wipe them down and oil the moving parts. If you try to polish them the polishing compound will get into the moving places and could cause unwanted wear.
Joe