They are all black and gunked up from not being cleaned by previous owner
Here's one way Sandy!
This link gives you 3 ways, Sandy:
spray with stainless steal appliance cleanser. let it soak for a few min., then wipe off. might need to do it again. and, the door opens so much easier.
just read the above two answers and I've done dit that.....not that good of results.
When I had sliding doors on tracks...I put Dawn dish soap and water in a spray or squirt bottle, sprayed or squirted the solution in the track and took an old tooth brush and cleaned and cleaned. Might need to soak for awhile, if it has been dirty for a long time. When clean, I rinsed it with clear water and wiped it out with a dry cloth. Some people said they used WD-40 on the wheels of the sliding door...I have never had to do that.
Soak paper towels in white cleaning vinegar and then let sit. That should clear up most of it. You can use a toothbrush to loosen if needed. QTips if you have tiny areas.