This is an "issue" that will never be definitively settled to the satisfaction of everyone.
There are several well known facts:
- RR363 is a DOT3/4 compliant fluid (I think it's now DOT 4, too)
- The only stated difference between RR363 and normal DOT3 fluid is the lubricating additive, widely acknowledged to be ethoxylated-propoxylated castor oil at percentages between about 8 and 10 or so (look up the MSDS)
- It is hygroscopic (like all DOT3 fluids) and really must be changed at a minimum of once every two years
- The company is never going to come up with an alternative nor endorse one. These cars have been out of production since 1980 and there is no reason on earth for them to do any R&D related to a brake/hydraulic system they haven't produced for just short of 35 years.
- Citroen owners have been experimenting with concoctions for a while now, and our systems are really not all that different from theirs
I personally know of:
- One person who's been using Prestone Synthetic DOT3 fluid exclusively for 8 years without any issues
- Three people who've been using a 90/10 DOT3 (or, in one case DOT4 Castrol GT LMA) brake fluid to pharmaceutical grade castor oil mix for 6, 5, and an unknown period of years without issue
I have conducted my own "on the shelf" experiment with a 90-10 DOT3-pharmaceutical castor oil mix for about 5 years to see if it would separate or coagulate under any conditions we normally see where I live (and, barring a polar vortex, we seldom reach even single digits F here). The stuff has remained entirely stable for that entire time in a bottle that allows me a clear view of what's happening.
This past spring when I did a rebuild of the accumulators on my Shadow II I decided that I had enough data to feel comfortable using the 90/10 mix discussed above.
The purists will insist that RR363 is the only factory recommended fluid, and they are right. There have been problems with supply of RR363 in the recent past and a "bad batch" that came out in the mid-2000s. In my mind, the writing is on the wall that RR363 is not going to be in perpetual production.
As is so common with any collector car past "a certain age" it becomes the task of the community of collectors to figure out "what works now" when so many fluids, lubricants, etc., that were recommended at the time of manufacture no longer exist.
You have to do your own homework, read quite a bit, and come to a decision you're comfortable with.
Brian, who also uses 2-EHA-free long life coolant in both my SY cars without issue (contrary to the roars of those who insist that only the old-type IAT "green stuff" is the only safe coolant to use) and the latest SN service rating motor oil, too (read the SAE and ILSAC specs and you know that this oil is perfectly fine for our cars)