There seem to be varying opinions as to whether flushing a slush-box is a good idea as opposed to just changing the oil & renewing the filter. There is a theory that flushing can disturb particles that are happy hiding in nook/crannies.
I'm in the same position with my daily driver, a 1998 E39 now with 148k on the clock & FSH, but I've no idea whether the fluid has been changed in the past. BMW state 'sealed for life' (which defies logic) whereas ZF recommend fluid changes every 50k or so.
The 'box (5hp19) functions perfectly so I'm going to have a fluid / filter change done next week & keep my fingers crossed
As norseman indicated, changing the fluid can cause debris to detach in the clean fluid and make itself known. Plus you'll only get about 80% of fluid removed without flushing. That's why people tend to give it a few thousand miles and then flush again. By that time 90% or so will be new.
The Debris "hiding" isn't necessarily true.
Debris will work it's way into a Valvebody if the oil is left or changed for new stuff, Debis/clutch material in the valvebody will cause the hydraulic fluid pressure to decrease and the clamping pressure on the clutches drops off and they start to slip..
Changing the oil regularly keeps the tranny healthy, as old fluid is used it ages and loses the lubrication and cooling effects, the clutch packs rapidly wear, which is the "black" fluid you get on knackered boxes, the black "sludge" in the bottom of the pan is clutch material, that some think is "dirt" lol
There is a pardox though, the reason flushing is considered bad i because on old knackered boxes the Black shít is clutch material, and paradoxically the floating material in the fluid helps the knacked clutches "engage" and when you flush and replace all the fluid with new fresh fluid, the material disappears and the clutches slip..
Odd things..
Dropping ths ump and replacing 40% is ok, it helps,
If the oil is black the tranny is usually ****ted.
Best wipe the old oil on a piece of paper, that'll give the true colour.
The smell is also telling..
A proper turbo 400 is what these big cars need