I have the same problem as of late..I know exactly what you are talking about ...but I do not think this is the oil issue.

I have a standard (5 speed) version) and on cold starts it runs very rough for about 5 minutes until slightly warm and then it runs perfect. (almost like it was running on 4 or 5 cylinders).

To me it sounds like the distributor cap or maybe even the spark plug wires ...which would maybe explain why this is not being picked up by the diagnostic system.

Anyone else out there that has had this problem PLEASE HELP!

Paul
Luxembourg