Lemme guess, this "expert" socal mercedes shop spends 85% of its time dealing with gasoline engines and that's what their reputation is founded on.

Lesser known fact, dirty diesel engine oil can have gelling problems same as diesel fuel. Their idea for straight 50wt this time of year in your diesel engine is formula for disaster. LOL, you'd even do better with straight 30wt instead of what those eggheads are recommending.

And just for grins if/when you go back to this joker at the socal mercedes shop, ask him if Pennzoil straight 50wt is diesel rated. And then ask him to xplain the difference between diesel rated oils and those that arent...... oops, the guy who recommended straight 50wt doesnt know.