There are people who regularly use 15W-40 in gas engines. The back of the container tells the story. Does Delo carry a "gas" rating? As of late, gas oils are rated SJ and more recently SL. Diesel oils have a rating that begins with the letter C.
It all comes down to what you want to use long term. Some believe that you should pick a brand of oil and stick with it. Different brands affect things like seals.
A local MB independent mechanic I know swears by 10W-40 for the 420/560 SELs and the M103 motors as well.
A veteran mechanic by the name of Randy Durrance(used to post here) once said "the best oil is clean oil".
1991 300-SEL - Model 126
M103 - SOHC
James Madison - Oppressors can tyrannize only when they achieve a standing army, an enslaved press, and a disarmed populace.
Liberty Firearms Training - After years of trying to regulate California criminals with a never ending stream of gun control laws they have still not given up with the belief that if they make something a crime that criminals will miraculously start obeying the law.