For normal application, for this trucks, you should use ISO - L - HM 68 or HLP ISO VG 68.
I would suggest to care about the quality of the oil.
The hydrostatic transmission is very sensitive.

Anyway, it's rather strange for me thet the truck from 2003 (over 10 years old) has only 3800 hours. It means it worked
~ 350-400 hours a year.
I would check the real working hours (ask Linde service to check the value from the control module)
I would'n t be suprised, if the real working hours number was 4-5 times bigger.

Besides, check the pressure form the control valve section responsible for lifting/lowering.
The maximum pressure value from the tandem pump is ~ 230 bar. It's a reduced in the control valve to 170-180 bar (depending upon the mast type etc.) But anyway, even 230 bar shouldn't be so disastrous for the mast cylinders.

From the other hand, if we consider, that the real working hours can be much bigger, perhaps the seals just reached their lifetime.