Best to stick with 5wt oil or if you need it max 7 1/2wt. Anything thicker will blow the puny reed valves under the piston[which are prob blown anyway!].
The cart should be full of oil but you must allow for the volume of the damper rod so make sure it is compressed into the cart when you have topped it up. Best to remove the circlip and the control valve to fill it-note the stud internally that provides the stop for the adjustment should be in the slot of the valve on reassembly.
Got all parts in stock if you need owt.
Hope that helps
I service and restore Pace forks