Two tablespoons of fluid on the ground after driving the car sounds like a lot. Assuming this is coming from the injection pump (IP), I'd say this is more than a tolerable leak.

There is no engine oil feed to the injection pump like there is on later engines, so there's only so much oil that could leak out before it's dry. If your car's IP is leaking a significant amount of fluid while running, then it's likely to be fuel rather than oil (or a mix of the two until the oil dilutes out and only fuel is left).

Fuel could leak into the IP's sump past worn injection elements. Some of this leaking is normal, and is why we must change the IP's oil at intervals. I do mine when I change the motor oil, and I use the same oil to fill. If the elements were worn enough to leak as much fluid as you are observing I'd guess that the engine was not running all that well.

Fuel could also leak into the IP's sump from the lift pump. I had this problem with the '74, which came down to a bad O-ring on the push rod that operates the lift pump. My car's symptom was excessive fuel coming from the drain pipe on the engine side (visible in Diesel911's post above). I corrected this problem when I rebuilt the lift pump. I still get some fuel in the oil, but not enough to cause it to drain from the pipe between oil changes.

Here's what the push rod and o-ring look like:


That photo is from this thread:
http://www.peachparts.com/shopforum/vintage-mercedes-forum/368673-w115-220d-240d-lift-pump-valve-rebuild-kit.html#post3674612

When changing the IP's oil you can't drain all the fluid out; there isn't a plug low enough. The idea of sucking it out with a baster or other tool is intriguing but I've never tried it. IIRC the book says to open the drain plug (see next paragraph) and pour fresh oil into the IP until the fluid coming out is mostly oil. The oil is more thinned with fuel when you first start pouring.

Here is a photo showing the LH side of one of these IPs. You can see the mount for the lift pump, pump removed. Then to the right of that is another hole which is where the drain plug threads in (plug not present). It's a hollow bolt for a banjo type fitting, so you don't have to remove it from the hole for it to drain. It's got a 14mm head.



Oil coming from the shut-off lever's shaft indicates a bad o-ring there. Also, if the drain pipe on the other side is adjusted high, the fluid level could be coming up to the shut-off shaft before it makes it up to the drain pipe.

In your shoes I'd open up the IP's drain plug, let the excess fluid out, and flush with fresh oil. After flushing with fresh oil close the plug and don't add more oil. Then run it and see how long it takes for it to start leaking again. And check that drain pipe per Diesel911's suggestion to make sure it's clear. If it starts leaking again soon after then you will be looking for the source, and if the car runs well chances are the excess fluid isn't coming from the elements. That would finger the lift pump as the primary culprit.

Hope this helps!