I have always made a point to check fluids at the same time, ie; with the eng. off. So, whenever I check the motor oil, I make a point to give the hyd. fluid a quick check. This is how I noticed the leel rising over time. I know for absolute fact that the level was dead on with the arrow on the hyd. res. the day I changed the fluid, and the days and weeks following. Today, 1 year and 4 months later, the level is at the Star on the hyd. res.
I have the W123 CD manual, and it infers that the pump can be rebuilt. If indeed the pump is drawing in oil, it would be through the o-ring that seperates the pump from the drive gear in the eng.'s head. I don't know if that o-ring is available. What I think I will do is draw out some fluid, to the full mark (arrow) and watch it more closely. The color and consistency of my motor oil hasn't changed, so I a presuming the hyd. fluid isn't getting into the engine. My real concern here is that the suspected mixing of hyd. fluid and oil will harm the rest of the system $$$$$$$.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member