I've been using the Lexol Leather Cleaner & Conditioner products (2 separate products) for years. Absolutely does a fantastic job. I had a 1986 Merkur XR4Ti with gray leather interior that I kept for nearly 20 years. Used Lexol on it the entire time I owned it. Leather upholstery always looked good as new.

For quick upholstery treatment (e.g., car shows), I use Meguiar's One Step Leather Care Cleaner & Conditioner. This one is in the deep red bottle, not the Gold Class product. Never had any issues with the product anywhere I used it.

For an applicator, I use a terry cloth or microfiber round applicator (like for waxing). Then I wipe off the product with either a terry cloth or microfiber towel.

I also treat my vehicles with leather treatment 3x per year (about every 4 months).