this creamy/mayo stuff on the oil cap is normal, especially in winter where you live in the snow belt. I see this stuff every winter. Interestingly though, it doesn't happen when I was using the dino oil, before MB mandate that FSS vehicles requires synthetic. But long story short, it's ok to see that. You will notice that in summer time or when the weather gets warmer, that stuff will disappear.

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98 ML320 (gone but not forgotten)