I am on the road now so I cannot tell you the brand name or quote the paperwork, but I have a couple of aftermarket transmission coolers at home. The paperwork with them specifically tell you not to install it as an oil cooler. the passages are too small and will restrict flow too much. They even give you the infomation about the line of oil coolers that they sell. They are all in the same ballpark for prices so I don't see any reason for them to lie. The oil coolers are generally 1/2 inch tubing with fins. The transmission coolers spread the fluid out into a series of a bunch of small passages.

I would replace it with a MB oil cooler or an aftermarket oil cooler of the appropriate size, IMHO.

-Jim

Edit: I misread your original email. I read Hydraulic as Transmission, sorry. What I said above about a tranny cooler still applies so I will leave it all there. As for yours, as long as the fluid passes through large enough tubing it should be fine.

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1995 S350D, Green with black leather interior.
Bought January 2008 w/ 233,xxx miles.
I did 22,000 miles during the first year of ownership.