I know this is an old topic but I got to ask. Do you slide between notes a lot? I tend to lose little patches of skin here and there from regular playing. Then I wrote a song that is nothing but sliding notes from one fret to another for like 3 minutes or so. Playing that one song just destroys my callouses because the round wound strings are acting like a rough file on my fingertips. If you do a lot of sliding up and down the strings you are going to lose some skin for sure. Hammer ons and pull offs for instance hardly do any damage at all. I used to be able to do those all day long without any damage. Oiling the strings just accelerates your skin damage in regards to sliding around a lot. I learned that the hard way. To be fair I used to drag my fingertips on everything everywhere I went when first started playing. Rough concrete, office style carpet, brick walls and whatever I could use to just rough them up while I was out and about and going through my day. I recommend at a minimum just taking more frequent breaks while practicing.