convertible top fluid

Yes... the top does use Mercedes Hydraulic Fluid
You can get it from any Ferrari dealer for about $35 or from Mercedes for about $24...

 

For the convertible top fluid, I use hydraulic jack oil. I have used it now for over a year and my top works better than ever. It only costs $5.00 and is available at any auto parts store. There is a detailed thread on Fchat on how to do this procedure properly. Use the search feature-it is priceless...

 

This is what I use,never had any problems, no leaks for over 5 years.
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Yes here is the link. I first heard (like many on here) about using jack oil from the guy who posted about it in this thread years ago. He did this detailed how to write up. http://ferrarichat.com/forum/showthread.php?t=128103&highlight=jack+oil&page=2

 

I would keep the fluid topped up near the max line. The Mercedes stuff is recommended by Ferrari, but as you can see, the system does not appear to be particularly sensitive to what is used. I use Pentosin CHF-11S, which is a bit easier to find, as a couple shops near me sell it here in Seattle. You never want it to run low, or you risk damaging the hydraulic rams. The previous owner of my car did that and paid plenty to get them replaced. The fluid really needs to be checked and topped up at least every couple years, and should be part of every major service. Surprising how many Ferrari dealers ignore checking the fluid until there is a failure in the system.

 

Curtis,
There are plenty of threads on diagnosing the "Slow Down" light on the 1996-1999 5.2 Motronic F355's, just use the search function in the upper left corner.