What you have to understand about leather is that it is effectively animal skin which isnt so different to human skin in many ways.
The dirt on leather is nearly always greasey type from sweat etc, now in the case of blue leather, that is VERY sensitive to chemicals for some reason and use the wrong stuff and it can bleach it, leaving run marks etc.
What I would do if I was presented with blue leather, is get hold of a baby shampoo, decent one, and try with that first with a nail brush gently all over, using kitchen roll to soak up the dirt. This is a great first step because use something too harsh and it can get complicated - if its in great condition this should be all thats needed. Otherwise I use diluted Autosmart G101 10%/90% water, but never leave it long enough to run.
Finishing the leather is where the skin bit comes in. I use Autoglym leather cream, mainly because people like to see a brand name with leather stuff, no idea why, but in actual fact, a good moisturiser performs the same task and I use this to great effect on my mums Hyundai leather which has remained very supple with a twice yearly application, her car now being 6 years old like yours.
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