The protectant I use is called OptiLens. I've had it on the W211 (E320 CDI) for the last four years, and so far, it's kept the headlights from yellowing again. I also used it on the 2003 S600's headlights in 2017, and it likewise did a good job of anti-yellowing protection. Good stuff that lasts for several years, and counting.
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