About "glow plugs" . . . I don't know about maintenance on those at all. Good question. Is there any preventive maintenance on Glow Plugs ?
When you get a lot of hours on them, they can develop a layer of soot, which causes them to loose efficiency.
So, cleaning them, can sometimes help. But, this is not normally necessary.
If you were having starting issues, you could pull one, and check it.
When they have a lot of carbon, they don't pull out without twisting, and pulling.
Usually at that point, you replace them.