I'll assume that we have the same injection pump or at least very similar...
As mentioned above, you don't have an IP that's lubricated from the engine oiling system. It has to be drained and replaced on a somewhat regular basis.
I had difficulties draining the old oil and mixed fuel out of my W115. I removed the red "Oel" cap and attached an oddball arrangement of small diameter tubing and electrical tape to my hand operated fluid pump. Eventually I was able to work it to the bottom of the IP and pumped out several quarts of old oil and diesel fuel. I wouldn't be surprised if you also find some diesel in the mix.
There is an overflow tube on the engine side of the IP that prevents it from getting too full of fuel that, hopefully, will never see any use.
Once you have the old crud out (either my method or working your way to the drain plug), you'll need to top it off.
There's an overfill plug on driver's side of the IP near the primer pump.
It can be tough to get too, but it's not impossible. A boxed wrench and a moderate amount of contorting should do the trick.
Loosen this and add about 1/2 quart (but probably less) of your favorite engine oil. Stop adding when it begins overflowing and retighten.
Rinse and repeat every other oil change or more often if you wish.