I would suspect a ring problem is causing the smoke. It's probably causing such blow-by at that speed/load condition that oil is being blown through the PCV system into the intake. Just a guess.
My wife drove a 126 body 280SEL (automatic) for about 13 years. (great car for her and my old college buddy who's wife currently drives it). We also had a 280TE for a shop car for the same years. The difference is engine noise and rpms at road speeds were significant. The SEL easily would go 100mph. The TE felt like it would blow at 80.
I'm not sure what ratio the 4spd carries but its likely that its lower than the automatic if you have those impressions. The TE had much better low end power and we used it to pull a 19ft ski boat that the SEL had a hard time with. A 5spd definitely allows both worlds; low end torque and high speed operation.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician