Wax. You can melt in paraffin or beeswax or carnauba wax with a hair dryer. Buff it out. Clean and dry, and won't harm the wood at all.
If you still have plenty of varnish on the surface of the wood, just use a paste wax. Renaissance wax is supposed to be good, though I haven't used any.
NEVER put any kind of nondrying oil on wood. NEVER. Contrary to popular belief, wood does not need to be "fed", nor does it need to be oiled to keep it from "drying out". Wood is perfectly happy being clean and dry.
Linseed oil (as it is found today) will not dry, for all intents and purposes (it will, but requires lots of direct sunlight, and must be applied in THIN coats). If it is slopped on, it will never dry, and the oil will soak into the fibers of the wood itself, turning it into soft, mushy goo. Gun oil is DEATH to wood. I hear so many people who say that they rub down the whole gun, wood and all, with an oily rag. Makes my skin crawl. Oh, how many old Model 11 shotguns have I worked on that have been propped up in a corner for years with the wood at the wrist of the stock soaking up gun oil...
Tru oil is a linseed oil based varnish, and I have had mixed results with it. Most of the time it will be good quality, sometimes it will produce a flaky, brittle finish. Just a bad luck bottle, I think. Tru Oil does NOT keep. Once it is opened, use what you need and you may as well throw the remainder away. It will harden up in the bottle.
A good spar varnish will work well, though I have found pretty much all modern varnishes will produce dull, lifeless looking wood.
Wood finishes is a subject near and dear to my heart. I build 18th century type flintlock guns, and make my own varnish.
