Hey Enzo,
After being a member of this site for many years and enduring the numerous discussions (ad nausium) re five speed gearboxes and reducing the revs at highway speed by new diffs and after recently completing a round trip of about 3000km at highway speeds, I offer this advice. Over 50 years ago, when those devilishly clever German engineers employed by Mercedes designed and refined the engine that is in our 230SL, they calculated a horrendous number of things - involving torque curves, rotational speed and a whole lot of other things that are way beyond my comprehension. In short, they designed an engine and drive train to perform at its optimum at those speeds. At 70mph, the engine is turning at about 3500rpm AND LOVING IT -that's what it was designed to do: for hours on end. Is it noisy compared to your 2013 Ford or Mercedes or whatever - sure is, ridiculously so, but hey, its 50 years old - its a classic. Now, to try and turn it into something its not, some people believe they can tinker with the calculations that those very clever engineers made (with their slide rules) and change the dynamics of the drive train because their sensitive hearing is offended.................... or they want to listen to classical music - the only music at that speed that needs to be heard is the roar/growl/howl of the engine
Get the picture - let it be loud and enjoy
cheers
PS - if you picked that this has become a bit of a hobby horse of mine - you are spot on.