Over the years I keep evolving and refining my methods and understanding of the process of oiling tool handles. Although it is painfully simple, the obvious is not always so obvious. I've been soaking my handles pretty deeply with oil for a long time, but still have had something of a fixation on coating them with a protective coat. Until, I realized that a well saturated handle is it's own finish, and more.
a Coating ona a handle is a barrier between the wood and the environment. But does it achieve that goal well, and what is the goal anyway? The goal is to protect the handle from environmental changes in moisture basically. Moisture swells the wood, and when it leaves, the wood shrinks. When wood shrinks, it is stressed and those stressed can lead to cracks. For some reason cracks seem more likely to form if the wood swells and shrinks repeatedly. If the wood swells within the eye of a tool, the wood compresses against the hard metal of the eye walls, becoming crushed. When it shrinks on drying again, it many shrink smaller, than it was before it expanded. That is why soaking the eye of a tool in water when it is loose will eventually make it even looser. A good thick coating of cured linseed oil can help prevent the entry of moisture, and anytime oil is used on a handle, some of it soaks into the wood to some depth, bringing in the factor of penetration, which must help some. A coating is basically still very thin though and will wear off over time. These are handles remember, They are essentially rubbed over and over again. And although some penetration is always occurring, the questions to ask is how much good is penetration when it is shallow and of a low saturation.
Enter Saturation. Saturation if you look it up, basically means full or at maximum capacity. But it is commonly used with a quantifier or clarification like partially, mostly, completely. If I soak a handle numerous times with linseed oil, it will penetrate to a certain depth, but unless it is applied regularly and in quantity, it will have a very low saturation as the oil spreads itself out deep into the wood structure. Eventually, it either reaches the middle or some unknown depth and starts to increase it's saturation eventually filling the wood to the point that no more will soak in. This 2 minute video shows the process I pretty much use now. If you get tired of adding expensive oil to a handle, try stopping for a month to let the oil in the handle cure and penetration should slow down. Some handles will take a lot of oil. Fortunately, oil is light.
Now if we think about a handle that is fully saturated with oil, for even 1/8 of an inch deep, let alone more, we now have something like the equivalent of a 1/8 inch coating. But even more cool, it is actually protecting the wood itself by filling the pores and structures that water would fill. If you leave such a handle out in the weather, water droplets just bead up on it and sit there. Not recommended, they aren't necessarily immune to moisture, but it's telling.
Try it on a handle and see what you think. It is a long process and the oil is not always cheap. many tools are also not subjected to much in the way of atmospheric changes, so it's not something we have to use everywhere. I'm pretty sold on it though and any axe that I plan to keep and use gets the full treatment now. Dudley cook recommends the same basically, but he maintains with an occasional coat, which I think is unnecessary if the wood on the outside of the handle is well saturated. The wood essentially becomes it's own finish. If the wood will ever take oil on and soak it up, do it, but it it doesn't, there is no need to keep coating it.
I use food grade linseed oil (usually labeled as flax oil, which is the same thing) anymore and have found ways to pick it up cheap enough. Boiled linseed oil is toxic and I think it probably dries too fast. Prices change on amazon constantly, but this brand is usually about the cheapest, but do your own research. I've also found flax oil at the local cheap food outlet where they send overstock and expiring stuff. Other oils can be used as well, walnut, hemp, poppyseed and tung oil should be adequate, but I really haven't used any of them enough to say for sure.
For handles that you don't need to saturate, I recommend a thin coat of oil once or twice a year, or better, just whenever you have an oily linseed rag. Raw linseed oil will cure, it just takes longer. So called "boiled linseed oil" contains metallic driers and solvents that speed curing time.
I have more ideas and experiments brewing around this problem, and no doubt you'll hear more about it in the future.