According to fueleconomy.gov on 1985 380SL, you should get 14 city and 17
highway. With an older car, there are alot of things that may cause lower
fuel mileage...leaky vacuum system... timing chain stretch... timing is
off...fuel injection leaking internally that you are burning too much
fuel... cat converter nearly plugged...
Your car takes regular and that all you should put in, you don't benefit
with higher octane gas.
I had 84 380SE, I always had 17or 18MPG regardless of highwy or city.