I usually buy genuine leather wallets an it seems like if you have them long enough, the cards you keep in them get dirty with some form of residue that doesn't come off easily.
Is there some way to clean it off without ruining the cards?
I'd rather ask before I try. 
I have a secret way to clean these - throw them in an envelope and mail them to me. 
JK - I too get all weird stuff on the cards in my wallet.
I just have my cards replaced when they stop reading. I get about 2 years on my main debit card before it no longer reads.
Oh they read, they just look like I pulled them out of the trash.
I keep my debit card in the paper sleeve my last one came in.. even with heavy usage, it lasts the 4 years till I get a new one
I get new credit cards and I laugh at how long they think it will last. In my wallet, most cards split into two pieces after about 2 years.
I use a money clip, so my cards see accelerated wear. At least it's a cool carbon fiber clip. Totally worth replacing all of my cards years sooner than I normally would.
Water usually works well at dissolving that goo.
I throw them through my washing machine in a pants pocket. Sometimes still in the wallet 
SyntheticBlinkerFluid wrote:
Oh they read, they just look like I pulled them out of the trash.
don't see what the problem is...
novaderrik wrote:
SyntheticBlinkerFluid wrote:
Oh they read, they just look like I pulled them out of the trash.
don't see what the problem is...
It just bugs the E36 M3 out of me.
Just a little soap and water, it won't harm the mag strip at all, but you might want to put some painter's tape over the part where your signature goes, that's usually some kind of sticker that could be damaged.
WD-40, followed by Windex to remove the oil.
Another fan of the tyvek sleeves here
wbjones
UltraDork
11/21/12 9:09 a.m.
but then they don't fit the pockets in the wallet 
Woody wrote:
Another fan of the tyvek sleeves here
My bank stopped giving them out.. so I have to keep using the same one over and over again
I just made a whole wallet out of Tyvek, no more bending or cracking of the cards and they stay clean.
mad_machine wrote:
Woody wrote:
Another fan of the tyvek sleeves here
My bank stopped giving them out.. so I have to keep using the same one over and over again
My Tyvek sleeve is 3 mergers old.
You guys are a lot more gentle on cards than me. I don't think I've ever gotten 2 years before they split. Scotch tape works quite well to put them back together.
You Yankee types got the chipped cards yet?