For anyone wanting to give their leather interior a good overhaul, I would highly recommend using Leatherique products. I use both the rejuvenator oil and their pristine clean once every three months or so, with another leather conditioner once a month. Leatherique isn't the cheapest products out there, but they are worth every penny.
The rejuvenator oil is amazing at bringing out the dirt, grime, air pollution, toxins etc from the leather. I'm amazed every time I use it.
And the Pristine Clean is absolutely the best leather conditioner that I've ever used. I applied this yesterday while it was hot out and took a before and after pic......although I don't think pics actually do it justice.
Before
After