There have been some excellent posts here and I've learned some new techniques and methods for cleaning and conditioning etc.
I wanted to bring to the attention a problem that I knew I was going to encounter with my inexpensive yet near mint Ted Simmons Rawlings MJ50 (made in Korea) mitt.
I knew from the minute I spotted that light tan (light palomino color) that cleaning this dried out mitt would be a problem. Any moisturizer or cleaner you applied would streak and darken the mitt and sure enough using a little petrolatum carefully resulted in this happening. This leather is basically an open pore, open knap type leather and transforms darker or streaks when any oil or moisturizer hits it. This inexpensive leather dries quickly over the years and is the devil to clean and renew the leather.
Which brings me to a point about the use of vaseline. Now Nocona has used vaseline for more than a half century to hand last its gloves in the post production process. I've used it for 40 years to clean and condition gloves. I had quite a discussion with "Glove Affairs" Noah Lieberman over his section in his book about the negatives of using vaseline. He told me he had gotten gloves from Dick Stump that were over vaselined and this IS a PROBLEM. Too much vaseline is NOT GOOD. Especially as mentioned previously in this area if you're playing with the glove as it picks up dirt and dust. Noah also claimed he talked with a leather expert who claimed that vaseline crosslinked with the leather and clogged the pores. I simply have not found this to have been the case.
Lanolin IS EXCELLENT to use.
Neatsfoot I have been told over and over by many, DO NOT USE as it DOES clog the leather pores. AND I've seen disastrous overuse of Neatsfoot in its various forms.