Transferred the whole gearbox to the correct side of the cases and wiped the mating surfaces down with a bit of brake-clean and a clean rag.
It really only takes a 1mm wide bead of RTV-sealant (I am using Dirko-HT - also known as the red plague overhere!)
And that's it. This is how it has been sitting in my dad's boiler room for about a week to dry.
As it was so cold, that even the oil got a bit stiff, I'll have to do the manual inspection of all gearbox shafts turning freely this weekend, because whereever the oil accumulated, it was as if I was spinning a shaft in grease...