I might be able to help here, having spent 35 years working with industrial wastewater treatments systems that included oil and grease removal. I have used all of the chemicals mentioned here. The DE will work but it is a very inert material and any oil removed will be from the oil sticking to particles of DE. DE comes is many grades ranging from very course to very fine. You might want to consider trying bentonite clay. Bentonite clay actually has an affinity for oil and grease very similar to a how a magnet works with a metal.
The other chemicals like potassium hydroxide (KOH, caustic potash) and sodium hydroxide (NaOH or caustic soda) work by braking O&G particles down into smaller particles and dispersing them in a solution such as water. Both are strongly alkaline. Wood ash is slightly alkaline and would therefore work well at removing some O&G as long as there is some moisture in the ash. A word of caution - highly alkaline (high pH) material can break down the cellulose in wood and make it brittle. That's not a good idea for a gun stock.
