First off don't go back to the light strings right away. When you moved to the medium strings did you lower the action from where it was with the light strings?

If not, you can unless it was too low already. Why? The lighter gauge the string, the more phsical amplitude the string will vibrate over. When a string vibrates it vibrates between the nut and the bridge back and forth. medium strings have less amplitude so the action can be lower...and you get more tone than with the light strings.

I suspect you kept the action up and with the medium strings are gripping tighter. Lighten up! I went from a "A" style to an "F" style when I bought my Loar and with the longer scale, the tension was less and the strings felt different, more rubbery so I had to play lighter. Took a while, but no big deal.

I play D'Addario strings. The heavier gauge phosphor bronze and they seem to last the longest. I wipe them after playing. While I don't play every day, I play on saturday's with a group doing traditional bluegrbutt and have no problem.

Now, for your fingers. If you've developed very thick callouses, I really think you're gripping too hard. Again, lighten up. Lower that nut if you have to, or go see a good luthier and have your mando "set-up". Probably $25 maybe more, it depends.

If your fingers still get sore, try soaking them in pickle juice. Yes, pickle juice. The "alum" that is in there tightens and hardens your skin. let it dry first of course, and wash off the vinegar. I 'd think Claussen might be the best.

I used to have the same problem when I started playing and I did just what I'm suggesting to you.

-- Regards, JP "The measure of a man is what he will do while expecting that he will get nothing in return!"