Directly quoted from Petros website:
In more than twenty-five years as a guitar maker and repairman, I have sought out quality products that pertain to guitar building, maintenance and restoration. Like most good luthiers, I’m a perfectionist and a stickler for efficiency and quality in my tools and products. To use inferior products is to waste time and money. Until now, the best products I have found were only available in industrial quantities. After receiving numerous requests from customers, I’ve decided to offer them to musicians in convenient and economical sizes. These are the same full strength, concentrated, industrial products I’ve been using on my guitars for years. These are the best products available to care for your guitar. Treat your guitar nicely and it will serve you well.
Fingerboard Oil: Fingerboards are not finished with surface coatings for reasons of feel, playability and wear. They are made of rosewood or ebony because these woods possess the desirable qualities of oiliness and wear resistance. However, because they are raw and exposed, their natural oils dry out and do not effectively resist the onslaught of moisture, salt and drying. This causes swelling, shrinking and cracking. Most wood preservatives are not effective on oily woods. Special oils are needed to penetrate these woods, combining with their natural oils to harden within the wood, not on the surface. Many finger board oils are just mineral oil which never dries, but leaves an oily mess that attracts dirt and grime. This is the only oil available that was specifically formulated for rosewood and ebony. Apply it 3 or 4 times per year for maximum protection.
Directions for use: Apply the Fingerboard Oil with a small piece of paper towel. Use enough oil to cover the board so it remains “wet” for a short time. Very old and dry boards will take more oil. Let it soak into the board for a few minutes then wipe the excess with a dry paper towel. Discard oily towels/rags appropriately and keep all products out of the reach of children.