Check with the tire shop on what they recommend for a tire with stronger sidewalls. I run 18`s on my 420SEL with 35 series tires, and I always have run at least 38-40 psi in the tires, it will ride a little harsher, and the rear tires will wear a little faster in the middle, but it is the best to protect the wheels and sidewalls. You do have a tire pressure monitor system on your SL, but as long as all 4 tires are with 3-4 psi of each other, it will not set it off. I have been running the high presure for several years now, and have not bent any wheels or damaged a tire yet, and I also go through L.A many times.