Is mercedes benz hydraulic fluid the only brand that can be used on a mercedes

1st look for visable leaks,there should be one if res is empty. there are aftermarket fluids that can be used just make sur they are compatible with your current fluid

+1 to find the reason the fluid leaked out in the first place. OEM hydraulic fluid (2 bottles) is available from FEBI/Bilstein, P/n #02615 through any of the online foreign parts providers; www.autohausaz.com comes to mind. Avoid the temptation to add a 'stop-leak' type of additive to the reservoir as it's been known to negatively interact with the hydraulic cylinder seals (11 of them) which will get very expensive. Consider having your hydraulic cylinders rebuilt instead for roughly 1/3 the price of new ones. The rebuilder I use is Dennis Ficken, 913-390-0506; quality work, fast turnaround, trustworthy and lengthy warranty.