Check at least once per year the oil level of the T-9 or Borg Warner gearbox oil. Both gearboxes feature a oil level plug. At the T-9, it is also important that the bolts of the transmission case cover are checked. The casing (especially during track use) can get pretty hot. Then the quality of the oil (above 90 degrees) deteriorates as a result of the lack of cooling. In principle, the gearboxes are lubricated “for life”, but high oil temperatures makes the gearbox less reliable. In the case of a somewhat stubborn shifting gearbox, there are also alternative available oils (such as Castrol or TSL) in order to improve the shifting.
Also check the tail slider piece of the gearbox for oil leakage at the oil seal. This could be a sign of a imbalanced drive shaft or worn out gearbox end bearings.
The Ford T9/ N gearbox leaks a lot. Remove all gaskets and replace after proper degreasing by DIRKO from Elring. Partno. 036.161 70ml. Grey and your worries are over!
Borg Warner/ T-5 service manual
The casing of the Ford differential is made of cast aluminium. Especially with the Donkervoorts equipped with more power (Cosworth, AGU and E-gas 210, 235, 270) and torque, it is recommended that the mounting/ fixation points of the differential in the chassis are checked on breakage in the casing / chassis fixation points caused by the forces released during firm acceleration.
Donkervoort now has a nice modification available. Please contact Lelystad.
Ford differential service manual