Under a microscope, leather's fibers resemble a network of fine steel wool. A good conditioner needs to penetrate deeply within this network, not just sit on the surface. Liquid blends of animal-based oils that mimic the fat liquors tanners use to supple stiff hides after they come out of the tanning solution help preserve the leather's original chemistry. Thick, greasy conditioners sit on the leather's surface and simply cannot penetrate as deeply as liquid products can. But avoid liquid products that contain petroleum distillates.
Petroleum is a solvent that can weaken leather structure over time.
There are some multi-use products that combine a cleaning agent with a conditioner. The problem with most one-step products is that while they may both clean and condition simultaneously, they don't do either one very well. They tend to condition only superficially and to leave residues. Choose separate cleaners and conditioners to do both jobs well. Leather polish helps your tack look showroom shiny for special occasions.