you can use 10wt or AW32 for this truck
as for the lifting problem you may want to check the pressure relief valve and make sure its set correctly, or that it doesnt have a broken spring inside it.
look at the pump you installed and see if it has a relief valve on it, some of the older trucks had this setup instead of putting it at the control valve like most newer systems do. If its not there then you will need to check the hydraulic control valve and locate it there.

It sounds to me like it just isnt holding pressure.
You could also have a cylinder issue but i'd look at the relief pressure first.