I have been running regular in my '69 280 SL for years. It has 9.5 compression ratio and on a compression check it reads 195PSI. There is no knock sensor. The timing is adjusted to the book. It doesn't knock or ping even on hard acceleration. If it did, I'd use premium. My '95 E320 has 9.2 compression ratio. I'm also running regular fuel. But I never rev the engine much over 2500. At 60MPH it turns about 2400. Now, when I take the car to Germany on the autobahn, I'll buy BV Aral. "Freie Fahrt für ein freies Volk"