Take this advice for what it cost you but my roof has been working great for over a year using something different than the M-B fluid.
Not necessarily being a fan or a believer in 'proprietary' or manufacturer specific stuff, I refilled mine with Sea Star hydraulic steering fluid used in my boat's steering system. It is also 'proprietary' but the system instructions say transmission fluid be can be used in an emergency.
I wouldn't hesitate to use either in a pinch. It's not a jet engine, it's a hydraulic system. I might flush the system later after everything was rebuilt.
Sea Star fluid can be found at most boat dealers if there is one in your area.
Since you have the trunk liner out remove the gray steel brace that covers the pump. It'll make it a lot easier to fill the pump.
Looked up the specs on Sea Star. Meets military spec H-5606G. Maybe it will work in a jet engine.
"MIL-H-5606G, MILITARY SPECIFICATION: HYDRAULIC FLUID, PETROLEUM BASE; AIRCRAFT, MISSILE, AND ORDNANCE (09 SEP 1994) [S/S BY MIL-PRF-5606H]., This specification describes the characteristics and provides the requirements for a petroleum base hydraulic fluid for use in the -54 Deg. C to +135 Deg. C temperature range (see 6.1). This fluid is identified by military symbol OHA and NATO Code No. H-515 (see 6.5)."