Properly cleaning the data side of DVDs (this article also applies to CDs) is one of the most disregarded hardware longetivity precautions. Often times, individuals wipe off grease and/or dust by simply rubbing the non-label side of a disc in a circular fashion, using harsh abrasives, such the underside of a t-shirt, or worse, a towel. Truth be told, that's not the way to do it! Yes, you may have temporarily fixed the read issue, but you have also permanently scarred the disc, which could cause many more read issues in the future. Why not take a few extra precautions...