On the 98 E300, the rate at which the steering rack leaks seems to vary. The Lucas product I put in helped considerably, but now the leak rate seems to be on the increase. Yes, I know I simply need to replace the rack. But I'm determined to delay as long as possible.

So....if I want to overfill the reservoir, do I simply need to make sure there's a bit of space in there when the fluid is at its hottest? I assume that overfilling to the point of no air space would result in too much pressure.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

... Which might result in blowing a seal. Maybe you should top it off immediately after a long windy drive.

Sixto
83 300SD

I suspect over-filling would simply result in an overflow of fluid from the reservoir. I don't know how tight the cap on that kind of reservoir is. Filling when hottest is certainly wise; you should be able to fill to close to the top of the neck.

Overfilling level depends on if the system has a vented or non vented cap.

A vented cap system can probably be overfilled by the distance between add and full. If the steering wheel is turned with the engine off, fluid might be pushed through the vent and make a mess. ( this is the limiting factor for overfilling )

A non vented system needs an air space to allow for fluid expansion ( heat ) , so I'd call 1/2 of the add to full distance. Late 90's GM front drive cars had a non vented cap and if the system was slightly overfilled, steering turned with engine off, it can push the return hose off the barb. ( as steering is turned, fluid is pumped out of the power cylinder and into the reservoir. )

A quick way to make a vented system is to pull the cap gasket.

At some point the rack may go 100% fail without warning so unless there are $ or time constraints, changing sooner rather than later is better.

Trying to get more miles from the existing one in hopes that the replacement will " last longer" is false economy especially once the cost of additives and fluid is taken into account. If the replacement lasts as long as the original, it isn't going to matter much if it lasts 320 K or 325 K assuming the car is still worth fixing at that point.