From my experience, I would suggest that it is extremely critical to use the correct hydraulic viscosity fluid in the 6.9's suspension system. There is a reason for saying this, BTW, I haven't checked the viscosity specification of the Mercedes (well, you normally buy FEBI aftermarket) supplied Hydraulic Fluid, but I would think it'd be close to the ISO 46 specification. Of course, you can also obtain ISO 32 as illustrated by Oversize and, in addition, there is another specification rating as well, although presently I am uncertain of the exact reading.
Obviously, I have had the opportunity to examine all suspension components in detail and I find that the levelling valves are unbelievable sensitive in their specifications and the work being performed by a number of components within each valve, but also the switching valve, the Pressure Regulator and the Hydraulic Pump as well. There are infinite steel ball sizes, different specifications of tiny steel springs, needle opening orifices, plus rubber and teflon rings and seals. It is all very precise, and you have only got to have one terribly small components in the system, and the suspension will just not perform the way it was designed to operate. With my levelling valves, I have a failure rate that is quite significant in the overall level of my efforts to supply exchange units and when testing in real life, you sometimes scratch your head because you just can't understand why a particular unit is not working the way it should. It is always back to the drawing board and you don't always know what you are looking for and where the problem is located. In closing, I would always recommend to use the genuine oil. I know it is much more expensive, but if you have a well maintained and a healthy working system, your oil requirements should comfortably last you for two years. Regards Styria
