The short answer to your question is yes, you must put premium in your 300 SE, hence the "Premium Unleaded Only" marker on your fuel gauge. How much damage you will cause in time if you use lower octance fuel is unknown to me. I hope I never find out. Maybe someone violated that recommendation and can shed some light.
My understanding is that the higher octance delays combustion, thus causing higher compression while the spark plug ignites enhancing power, eliminating "knock", and decreasing causes of engine damage. This holds true for high performance sports cars and luxury cars. Premium fuels for other cars may be just a waste of money. Consult with your manual.
Continuous usage of low octance (87) fuel will eventually cause piston ring damage sooner as compared to the correct (92-94) octane fuel recommended. Remember these cars go 150 + mph on the Autobahn.
I am interested if anyone tried to bypass this and what kind of performance they got with 89 and lower octane fuels. I have not risked it and always spend the extra and get the premium.
'89 420 SEL
'90 300 SEL
'68 Olds 88 Convertible
'84 300 SD (sold it)