Hi Jonas, I'm finding my self tuning in each day now for the next episode of "oily speedo", desperately hoping for another clue to help us find the culprit!......
I have had a thought which could be a possible cause, or if nothing else one more thing to eliminate. Oil foaming, I'm guessing that when you changed the gearbox you would have also changed the gearbox oil. Is it definitely the correct oil? Not just the correct viscosity but also the correct specification, modern oils have lots of varying characteristics, for example some oils will have anti foaming additives, others high detergent qualities, others may be be particularly suitable for brass synchro, (GL4 for brass not GL5).
I'm fairly new to the world of AC's but I'm sure there is someone on the forum who could let you know the best oil to use.
Hope this is of help.
Nick