I'm not sure if the same problem can arise from the 320 or 500sl. I originally thought the cover might be some sort of a composite because of what look like plastic decomposing around the where the wiring cover attaches to the valve cover. I had an oil leak, so I ordered in a cover seal kit for my sl. I was going to clean up the cover and paint at the same time. As it turned out the cover is all alloy and it looks like electrolysis or bad wiring arcing may have turned the alloy to a powdery paste. strange that it only effected three of the four attaching spots and one of the bolt holes that attach the cover to to the head. one spot had ate completely through, I'm trying JB weld on the hole, I also tried to attach some shelving brackets to the cover using the JB weld. I used e-clips on the brackets to attach the wiring cover to the valve cover. the JB weld worked good on the steel brackets, but not on the alloy of the cover. my next step is to etch the alloy and try the weld again. no rush because the cover is just decoration and is held on just fine with one screw. I don't know if the foil covering the valve cover was part of the electrolysis problem. luckily the foil type finish with age comes off fairly easy. after going down to metal I used a high temp primer, wet sanded until close to perfect, and then finished with a high temp semi-gloss black engine paint. looks far better now and the cover doesn't leak on to the exhaust manifold. stinky. I would remove your wiring cover and run in the dark to see if the wiring is arcing to the valve cover and check the attaching points of the wiring cover for deterioration. this could also apply to sl320 and I don't know if there could be an issue with the 500