Just to add to Coffee pot, not only is heavy lube or grease needed on the rails to prevent wear of the slide, the reverse is true as well. The rails themselves will lose that coating if not lubed enough.
As a proponent of the p89 and especially the p93, I have seen this numerous times first hand. It's usually the front two rails that have that oxide coating rub off leaving bare aluminum underneath.
Then you have a steel slide riding on top of softer aluminum.
Probably not a good combination long term though I have yet to have functional problems and I have a couple rails just about rubbed off of coating, and the slide wasn't affected at all.
That is the only drawback to these pistols I have found, and is a big reason the p95 and the like are good, those polymer rails don't hardly wear at all and require no extra attention.
I have both, and just picked up another P95 for this very reason really.
I still love my 93's, but the 95 is rapidly becoming my gun of choice.