Oil would make the clutch slip.
I'm assuming you have checked the clutch plate operation via the inspection plate.
There are various things that could be your issue
Type of gearbox oil used
Synchro
Clutch release bearing
Clutch plate and diaphragm
Gearbox input bearing to crankshaft.
Usually the gearbox can be removed and leave engine in car though a lot of the interior may have to be removed.
Someone with an Aceca / who's done it before will hopefully answer your question.
You don't say if it's a standard or lightweight gearbox.
Graham
I'm assuming you have checked the clutch plate operation via the inspection plate.
There are various things that could be your issue
Type of gearbox oil used
Synchro
Clutch release bearing
Clutch plate and diaphragm
Gearbox input bearing to crankshaft.
Usually the gearbox can be removed and leave engine in car though a lot of the interior may have to be removed.
Someone with an Aceca / who's done it before will hopefully answer your question.
You don't say if it's a standard or lightweight gearbox.
Graham