Where do you get blue metal ones?

Also what weight oil are you using?

I use 10-30W dino per the MB dealer's recommendation and have had no problem. I would like to go to a heavier summer oil - some use 20/50W . THat scares me as I believe that these tubes may blow out their plugs as a result of high oil pressure in a cold engine. The difference in oil pressure between the weights of oil is significant on a cold start, perhaps double. Normally I'd use 10-40W year round but not so far in my Benz.