Dirt nailed it.

I've found over the years you can have jobs where GAL rollers have been installed for 20 years and eventually the doors will start to squeal when opening and closing. And I've learned in those cases (rollers been in service for years) you can use needle nose pliers and yank the oilers off and wah lah! Quiet doors! And in that case the roller will keep on working just fine for some time to come but quiet now.

It's because of that revelation coupled with I just so happened a few years ago to have a lot of GAL equipment needing rollers that I thought well I'll outsmart them, I'll leave the oilers off because in 10-20 years they'll be squeaking and making noise. So for a little while I didn't put oilers on with new rollers. Then it wasn't long I noticed the rollers I'd recently done were getting really rough, and like you said like a black coating flaking off, plastic of the roller noticeably rough. I had to replace some rollers that were less than a year old.

You definitely need to put the oilers on when installing rollers. And oil them with either motor oil or silicone spray. I avoid wd40 as it gets sticky over time. When I've replaced rollers with oilers installed they've been perfect. And in 10-20 years I'll just pull the oilers off.