It is very common for a Simms pump to drain more oil than you put in. This is because as the injection plungers and the bores they run in wear-- and they all do, it's the nature of the beast-- fuel will begin to leak down past the plungers and into the lube oil down below.
The lube oil simply lubes the pump's drive mechanism, governor, etc. The injection plungers are lubed ONLY by the lubricity in the fuel. This is why a lot of FL120 operators, including us, use a lubricity additive in their fuel. Whether or not today's diesel fuel has sufficient lubricity for old-technology engines like the Ford Dorset is a matter of hot debate. Our diesel shop and friends of ours in the marine diesel and generator manufacturing industry say no, and strongly recommend the use of a lubricity additive in an old jerk-injection engine like the Ford Dorset/FL120. Others disagree. We figure better safe than sorry.
Anyway, as the Minemec/CAV/Simns pump injection plungers and bores continue to wear over time, more and more fuel will leak down past the plungers into the lube oil down below.
This is why if you fill the pump with new oil to the level plug and 50 hours later you remove the level plug, oil can come out. What you are seeing is the volume of leak-down fuel during the last 50 hours.
Eventually the wear will reach the point where the leak-down is significant and the pump will need to be rebuilt. Usually by that time the injection system will be far enough out of whack that the engine's operation will be suffering, too.
Every engine is different and every operator will have a different manner of treating the engine, from cold-start technique to warmup to acceleration to cruise rpm. Plus there is the issue of sufficient lubricity in the fuel going through the pump. So it's impossible to come up with a one-size-fits-all interval between injection pump rebuilds. Might be many thousands of hours for some operators, might be only a few for others.