You're not "feeding" leather when putting oil on it and not every leather project has oil in it.
Quick run-down of the leather making process:
1) Cow dies and goes to the processing plant where the carcass is removed.
2) Carcass is sent to a tannery (leather processing company).
3) Carcass is chemically cleaned to remove all hair and other "stuff".
4) That is called raw-hide - duh, it's a raw hide. They may put some chemicals in at that stage to prevent decay, I don't work with raw-hide so don't remember.
5) From there it is processed in MANY different ways via vegetable tanning, chrome tanning, etc. At this stage they use tannins (where tanning comes from) to prevent degradation and at this point it officially becomes leather.
Vegetable tanning (veg-tan leather) uses vegetable oils and a myriad of other chemicals chrome tanning (chromium is used, as well as other stuff). Some people also say "oil tanned leather" but that's a misnomer, it's still veg-tan leather they just use a variety of different oils during the process, the leather is literally washed and then ran through a ringer to excess fluids out. If you have a soft leather in your hand and bend/twist/pull it and the colors change then this leather has more oil than others and is known as "pull-up" leather, as you pull it the color changes.
Chrome tanning is generally done in the fashion and furniture industries as the process is much shorter than veg-tan leather. Veg-tan leather comes in different varieties as well - died, natural, oiled, etc. From there, you start picking different parts of the hide for specific projects - top of the hide (near the spine) is good for belts (strength) and the best belt leather. English Bridle leather is considered the best for belts as it doesn't stretch that much - cheap belts use stitching to prevent stretch. Shoulders and other parts are good for bags as it is still pretty strong but more flexible. The bellies are used for small items such as purses, wallets, etc. because is soft and supple.
And what happens over time is those impregnated oils seep out, get washed out, or otherwise dry up and your nice, expensive, leather product dries up and cracks.
That is why you should always oil your leather if you plan on keeping it for years.
Different dying and finishing processes also change the way leather feels. Oil/alcohol based dyes will dry out the leather so after dying it's best to use some sort of finish to not only prevent color seepage to the user but also put oils back into the leather so it doesn't crack. If you have leather products that are old you would be better ahead if you used an oil designed for your product so it doesn't eventually dry and fall apart. Mink oil is/was the standard for a long time but it has it's limits. Water based leathers don't it out as much but the initial feel (at least to me) is it's more stiff than alcohol dye but if you flex it around it will loosen up.
I'm sure there are more knowledgeable guys here but, I got into leather about a year ago and have really began to enjoy it, so much I am considering selling my pool table so I can have an actual leather working room
This timing of this post is funny ironic because I actually registered my leather website last night
My pool table is currently filled with 3 sides of leather - 1 still intact, mostly, and the other 2 have been cut up into manageable sizes and I'm working on a jig so I don't have to sit there and cut crap all day. My kitchen counter is currently covered in saran wrap with paper on top as I test out different dye types, colors, etc. as well different finishing technics for the look I'm going for.
EDIT: I forgot this was about tips
I looked at the process of making milk-duds a few years ago and I do remember some chatter about the chemical in milk but after trying a couple of milk-duds myself I determined they simply weren't for me. They were medium tips though and I prefer a hard tip but I didn't feel the need to research it any further.
I would highly recommend you do NOT put any type of leather oil on a tip, that stuff will leech out and if it doesn't turn the chalk into a paste it will at least transfer to the CB and any English you attempted would be null and void.
Leather dye might be a different situation, both alcohol and water based, but not sure I care enough to look into it