The weather is not all that cold in Orange County so your oil will not gel but the problem is straight non diesel rated oil doesn't have the additives that keep soot in suspension so I don't know whether your lifters are going to like it or not.
You should avoid that mechanic and use 15W40 synthetic in your 603 engine.
With 411K on it I'll wager the rings are a tad worn (mine were at 260K) How much oil is the car using?

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!