marc
you best be sure what you have in there before just throwing any ole fluid in it.
jungheinrich doesnt use regular oil in most cases
they use mineral oil type fluids for hydraulic oil and brake fluids and in gearbox fluids for transmissions etc they use Shell Spirax MA80W.
If you have the owner/operator manual for this unit it should have a breakdown of what fluids go in this unit in the maintenance section. In some cases mixing oil's would be a bad idea. There are some folks that may tell you it wont hurt to do it but trust me, in the long run you dont want to have to deal with the damage it might cause. Jungheinrich is like linde when it comes to thier oils, they are specifically designed and in most cases unique to thier products.