Congrats on your 15-91! I love mine! You mentioned you bought Singer motor lubrication. It's not a good idea to put the new motor lubrication that Singer now makes into the 15-91 grease pots. It has a higher temperature melting point, vs the old grease. If you have a vintage tube, great! If not petroleum jelly works, I heard. I'm sure someone hear knows more than I do.
Cari is correct about continual leatherwork on the 15-91. While it's a great machine with a lot of power, if it is sewing thick leather all the time, you run the risk of burning out your motor or stripping your gearworm. The motor and mechanics were designed for light/home sewing. If you really want to sew leather, I have seen vintage industrial Singers, like the 96-80, for sale on Craigslist for about $300.
ETA, Leather sewing bags is light and all not hard a machine, but shoe leather is far thicker. I know of a man who sees leather bags on vintage machines, so maybe that's what you saw on YouTube?
Last edited by NZquilter; 01-14-2018 at 05:12 PM.