I was cleaning the ‘98 Legacy GT Limited today and noticed some very small cracks starting in the leather upholstery. What is a good product to use on older Subaru leather seats to keep them from drying out. This is my first car with leather seats

If the leather is already cracking it's too late to reverse it.  The "leather" in older Subarus is not leather, in many cases.  I know the OEM "leather" in my wife's '99 Forester S has fabric glued to the back, if that tells you anything...

Any leather cleaner/protector from a reputable brand will be fine, just follow the directions.  If you do indeed have fake leather, just use vinyl or automotive interior cleaner on it.

The leather driver seat in our '04 Outback VDC was very badly cracked when we bought it. I assumed I'd have to buy another seat...

I bought some Lexol cleaner and conditioner (the only brand in stock at Wal-Mart at the time that had separate cleaner/conditioner products). It rejuvenated the leather considerably, and it still looks pretty good (doesn't look like new, cracks are still visible, but far less evident, and haven't gotten any worse) 4 years and ~70k miles later.

 

Now, it's the only product I've ever used, but I still have the same bottle, and use it regularly on that Outback and our conversion van. And will probably buy the same again when this is gone.