Once the baked enamel coating starts to crack or chip out, you can get snags in your clothing from the rough edges. Not a good sign at 5-years. Some of them have SS, and thus no enamel on them, a worthwhile upgrade. Take things like jeans with metal buttons, and turn them inside out. Depending on the model, on some, a couple of screws and you can take the front cover off. Then, you can see most everywhere and use a brush or vacuum to clean things up. On my old one, the idler rollers used plain bronze bearings. the one near the burner would get all of the oil burned out of it. A drop or two of oil on the bearing material can make that one last years longer. You can also adjust their position to keep slop out of the drum, which makes it run quieter and the bearings and belt last longer. They're fairly inexpensive, so you could also just replace it.
On the duct, first thing is try to use smooth walled stuff. Don't think of using the plastic corregated stuff. You'll end up with lots of lint, slower air movement, and longer drying times plus, with the lint, a bigger ignition chance.
An air compressor can blow out a lot of loose crud in the duct. I installed a new dryer recently, and did this. I didn't get a whole bunch of stuff out, but it was worthwhile.