For short term care where the gun is in use (longer term if in is lightly used), use wax. Renesance Wax is the paste of choice for most (synthetic with known neutral Ph), but plain 'ole Johnson's Paste Floor Wax did good service for me for 20 years.
Agree with CH that lacquor is better for the long run. True Oil will work, is more durable than wax, and less durable than lacquor. I wouldn't bother with shellac.
True Oil over any stock finish will start forming a new wear surface - no damage. Many guns with an oil finish can be brought back to life by rubbing out and cleaning the old oil and adding new.
