If during your 60-mile day the car never smooths out, I'd say it's time to look for problems other than oil on your car. On a cold morning, a healthy engine with a reasonable oil grade should stumble for no more than 5-15 seconds on startup. Your car seems to have a driveability issue. Going from 5W- to 0W- won't solve this issue. How is the tune of the car?

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95 E320 Cabriolet, 169K