The teeth or ribs on the 300TE were so worn out I could not imagine it still functioned properly, albeit with a bit more noise. Some sections were completely gone (bald spots) and almost every inch had more than 4 cracks. Purchased the car at 223k miles and replaced the belt at 270k miles without knowing when it was last done.
On the 560SEL I think it's a V-Belt instead of a Serpentine belt. I would check the alignment of accessory pulleys, idler arm pulleys and condition of the tensioners.