The fluctuation in RPM is a common characteristic of the 5HP gearboxes and is supposedly down to the torque converter - a couple of American transmission specialists offer a modified converter to solve it. My S8 does it too at ~1800rpm, just before the converter clutch locks up. Its highly annoying but I just put it in S if it bothers me. Its curious this started for you after a fluid change though. Febi 29738 is the correct specification.
I still haven't figured out why some gearboxes are just fine after a fluid change and some aren't. I have a feeling it could be down to the re-filling period when the level is some 3-litres short due to the stupid design of the filling solution, although I couldn't say what specifically gets upset during that time.
I was going to contrive a device which could get the full load of oil in with the engine stopped so that it didn't have this (albeit short) period of running low on oil, but I've been working on the manual swap instead
The suggestion to refill, run it for a bit then drain and refill again is good, however since the gearbox has to be at 40° to set the level it would be an overnight job so that it could cool down. Still, there are a couple of nice hotels near the workshop
