I'll pipe in to agree with you on a couple of things. First of all, I'm a lifelong fan of Castrol Oil products and believe the final formulation of their oils to be superior to Mobil's. I have testimony from many professional techs and fellow afficionados who've experienced the same results using Castrol.
My question to you is why didn't you use Syntec? I've noticed the same noiser valve train sound at idle after the dealer performs the scheduled maintenance. Since I do my own services every 4K miles, I switch back to Syntec 20-50(S.Cali) and the Mobil-1 "effect" goes away.
But in your case, there is another factor; it's pretty well documented that cars with your mileage sometimes develop abnormal symptoms when switching to synthetic oil late in life.
I've read about minor leaks, some burning...etc. I buy synthetic because it is better formulated oil than Dino, but I also believe that I would be fine using Castrol GTX and changing it every 3K miles....
Ed "Don't Benz's just feel better..."
2000 ML430 Skyview "The Mel"
2000 CLK430 Cab "The Cab"
85 300D Turbo "The Diesel"
"One should as a rule respect public opinion in so far as is necessary to
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this is voluntary submission to an unnecessary tyranny, and is likely to
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