Like all things, they degrade over time. It is not unusual to have to change the leads within 2 years/40k on some cars. It's usually best to have a good look at them and if you can see any wear marks, dark spots or grey/arc spots, then change them. It is good practice to replace all of them and eliminate the hassle of any others failing. As far as what brand, well genuine leads are actually a decent quality and they fit properly. You can try some other leads but you will not get any more power out of them and they will still only last about the same as factory ones... 2c worth.