Our harness leather is fairly well protected with wax and tallow that is in the leather from the tanning process, but may need some additional conditioner after some use. You can use any leather cleaner + conditioner as it will moisturizes and protect the leather.
For the most part, you want to keep water and dirt away (but who are we kidding these are dogs wearing it so we know that's hard to avoid) while making sure the leather doesn't dry out which could lead to cracking. It takes a long time (years) to get to a point of drying and cracking, so you shouldn't worry about that for awhile. If down the road you find any leather of yours getting very dry, a coat of mink or neatsfoot oil will also help bring it back.