I used 10W-30 conventional for the first 3 years of ownership in Europe (even back then, synthetic was hugely expensive in NL). Switched to synthetic when I arrived back in the U.S. in 2001. 10W-30 syn (Castrol Syntec) in VA both summer and winter. Switched to 5W-30 (Castrol, BMW OE or Mobil 1, whatever is cheapest at the time) here in AZ year 'round, including lots of track time with the DASC. (fingers crossed here)...no burning between 5K changes and no blowby at 100K miles (the last 25K w/ DASC).
...did I say I LOVE this car?
