Here's what Les Tam has to say about using Neatsfoot Oil (quoted from his website):
"Les on Neatsfoot
I've heard a few times from people who have used Neatsfoot oil on their leather and claim that it had ill effects on their leather. Below are some of my thoughts on their claims.
I have not had ill effects by using 100% Neatsfoot oil on the Hermann Oak strap leather that I have used to make slings with for the past 18 years. Then, the question arises as to why it is that those people have problems and I do not have problems. I suspect that those problems arose from other factors, like:
a. Using poor quality leather from a tannery that does not have a quality tanning process. Such is the case if the leather was tanned in Mexico or from some other specific tanneries in the US which I will not mention. I only use "strap leather" from the Hermann Oak Tannery in St. Louis, MO. That tannery has a good reputation for producing quality leather. Although their prices are significantly higher, their quality is hard to beat.
b. Using products to treat the leather which contain petroleum derivatives Those products tend to gradually break down the fibers of the leather and destroy the quality and strength of the leather over time.
c. Using too much Neatsfoot oil on the leather. Too much Neatsfoot oil will cause the leather to rot and will cause excessive stretch. One of the old Army procedures from WWI and WWII was to take new untreated leather slings and soak them in Neatsfoot oil and then hang them up to stretch out by attaching a full ammo can to the sling. That method produced excessive oiling and caused the slings to rot and stretch out excessively. It is important to use only the correct amount of Neatsfoot oil at reasonable intervals (a light coat of oil about once a year application) for proper maintenance of the sling.
d. Using the correct type of leather (the tanning process) for making the sling.
For making my slings I have found that "strap leather" produces the best slings in terms of the correct amount of stiffness and stretch. There is a type of leather called "sling leather", which is advertised for making slings, but is, in my opinion, totally unsatisfactory for making slings because of excessive stretching of the leather caused by the amount of oils and waxes used in the tanning process. A 54 inch long by 1 1/4 inch wide strap of that leather will take a permanent stretch of about 4 inches with the first day's use for a competitive shooter. There are a couple of sling makers who use "bridle leather" and "Latigo leather" to make their slings. Those two types of leather, in my opinion, are not suitable for making top notch slings for the competitive shooter because they stretch too much, even though they are good quality and soft and comfortable with initial use. I suspect that the excessive stretching is caused by the amount of oils and waxes that go into those types of particular tanning processes. For my slings, the tanning process used to produce "strap leather" has the right amount of stretch or "give" and strength for use on a competitive rifle sling. Different tanning processes have been developed to produce different leathers according to desired/intended use. A sling maker of competitive rifle slings needs to select the most appropriate type of tanned leather that will produce not just a good sling, but a great sling.
Hope these above thoughts give you better insight into the area of making not just good slings, but "great slings". Of course, the final proof of the pudding is to shoot with the sling and see how it performs. But before making any judgment on which sling is better than the other, one has to have the experience of having tried slings made by different makers. One can also talk with shooters who have tried various slings and get their opinions/comments on performance and longevity.
Les Tam"