All SC300 and 400 motors are equiped with engine knock sensors. These sensors !Removed! engine timing to prevent low octane fuel predetonation from damaging the engine. Retarded engine timing means lower performance and lower gas mileage. Soooo.... save $ at the pump and get less HP and lower MPG. Not easy to quantify, but likely an even trade off.
I have been using 87 Octane for the past 2 yrs in my SC400 and I have not had any problems at all with my engine..nor have I had any problems with performance....the only thing I have found with premium gas is...burns faster and hotter then regular and less miles per gallon..just like E-85 does in the newer cars..with automatic timing there shouldn't be a problem...and by the way I have had the heads off the engine to check to see if there was any build up of carbon in the engine...NONE..as far as I can see the Oil companies are in with the automakers and want you to burn High grade gas because it makes them more money..the bottom line...the only cars that need premium gas are cars like formula 1's, Ferraris etc. I've had many mechanics tell me this over the years...but I was so afraid to try it thinking it would ruin my engine, but like I said with the New automatic timing in cars controlled by your ECM, shouldn't be a problem.. It's ok if you don't believe me..it's your car, Im just telling you my experience. Good Luck