When it comes to leather (or suede especially), any type of spot, spill or discoloration is extremely difficult to remove or even reduce in intensity.
The type of leather, the way it has been tanned, finished, and dyed, all affect its care and ability to be cleaned.
Generally speaking, oil is the worst of the culprits as far as difficulty of leather cleaning is concerned, followed by liquid and water stains and then paint.
Oils, even when stripped out and then dyed over are often capable of coming back to the surface in a year or two (hair oil is the biggest problem in this area).
We have specific chemicals, tools, supplies and procedures to deal with every type of problem with leather cleaning and our best technique is to have you bring the worst cushion to our plant first for testing (some cushions are attached so other steps have to be taken).
Just like with area rugs, we always consider costs versus value when something involved is apparent.