Saw this on www.thecarconnection.com.
I suppose the other question would be "it may not hurt the engine but will it make it run better?" Their answer I suppose is that Bosch guarantees your car will run better on Plat +4's
Q--Recently it was time for the 60,000-mile service in my ’99 Mercedes E-Class wagon. I asked the dealer if they could install the Bosch Platinum+4 plugs. They said only the original platinum plug was recommended. I called Mercedes’ technical service and they said it would hurt the engine because the +4 plugs run hotter. I e-mailed Bosch with the question about using the plug — after all, they have a part number for my car — but have not received a reply. Perhaps you could forward this question on to Otto Stefaner at Bosch to see if he can shed some light on this mystery? Is it possible to get a confirming note from Mercedes?
A--We did as you asked and here is what Mr. Stefaner has to say: “We cannot expect the vehicle manufacturer's technical service people to recommend spark plugs that do not show their company’s brand. The explanation, however, that Platinum+4 spark plugs run hotter, is incorrect. Just like other spark plugs, Platinum+4 come in different heat ranges. That means there are colder versions available for engines that require colder plugs, and hotter ones for engines that need them hotter. Bosch determines which Platinum+4 has the proper heat range for a specific engine, and that's the one we recommend. This way, the recommended spark plugs won’t hurt the engine.”