Well, clearly in the 1980 mainfold you burn all blowby. In the 1983 manifold you get some ability to recover liquid oil and return it to the crankcase.

On the W123 300D (Turbo) this function takes place in the air filter housing, which is considerably cooler than the intake manifold. But liquid oil probably isn't going to burn all that much while still in the 1983 240D manifold.

You'd have to think the 1983 manifold is a bit of a design improvement - although it may be mostly with the idea of reducing emissions. If your car has minimal blowby then either manifold should work fine. I see you have the 1980 one bolted up already.

Why don't you go with that one (1980) at first and see if you get much oil smoke after breakin? It seems the 1983 manifold would come into use later in an engine's life when there is more blowby.

You probably plan to keep the 1983 manifold - so its available if something unwanted starts to happen with the 1980 manifold. Of course I would expect there is an unused return hole to the oil pan you have to plug?

Ken300D