I have used Howard Restor-A-Finish on Mil Surps with good results.
Howard Restor-A-Finish
This dissolves the finish and spreads it around. It will take the crud off and leave the original color. I have fixed cracked stocks then used this to spread the finish over the repairs.
It is available at Lowes and Home Depot.
They also make another product,
http://www.howardproducts.com/feednwax.htm that I have used on stocks it is a combination of beeswax and carnauba wax.
Do not use these products indoors.
If you want to test the cleaner before you put it on visible wood, try a little under the barrel or under the butt plate.