The gear oil viscosity grade of D4 is 70W80. I would try a slightly thicker oil if the noise increases when hot, which the next step up would be MTL. Red Line recently released this MT oil that meets MB 235.10 spec of OE "manual transmission fluid", but it looks to be thinner than either.
https://www.redlineoil.com/mt-lv-70w75w
Here is additional reading on why a MT specific fluids can be a superior option than DexIII ATF.
https://www.bobistheoilguy.com/forums/ubbthreads.php/topics/5144795/re-synchromesh-manual-transmission-lubricants#Post5144795