I just did my 1987 300TD a few week ago.
I used a feed and flush method and it is not a 100% replacement of the fluid as there is still some in the spheres and rear shocks.
First, only use the MB special hydraulic fluid.
I just attached a small flexible hose to the return like on the top of the reservoir and start the engine. The other end of the attached hose was in a milk jar and you can see the discolored fluid coming out. I waited until the reservoir was almost empty and dumped in a liter of the MB fluid. When I saw the fluid coming out was clear I stopped the engine. It takes about another liter to bring the reservoir level up to the max level. It took a total of 2 liters.
The reservoir itself is disclosed (plastic) so even the fluid inside is clear it may still look dirty from the outside.