DOT3 & 4 can safely be mixed. HOWEVER DOT5 (or 5.1) brake fluid is silicone based and must NEVER be mixed with DOT3 or 4.

I run Castrol Dot 4 in my Westy.
https://www.summitracing.com/parts/csx-1261412504?seid=srese1&cm_mmc=pla-google-_-shopping-_-srese1-_-castrol&gclid=EAIaIQobChMIuKzX9ozt2gIVW7nACh3BPw6WEAYYAyABEgJWKvD_BwE

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of David McNeely
Sent: Friday, May 4, 2018 2:37 PM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Brake fluid

IIRC (but check the owner's manual), the correct brake fluid for the
Vanagon is DOT 3.  It matters, because they have different volatility, and
different ignition temperatures.  Whether they are mutually soluble or not,
I am not certain, but I would not mix them, just as I would use the type
recommended by VW.

On Fri, May 4, 2018 at 9:27 AM, ddbjorkman@verizon.net <
ddbjorkman@verizon.net> wrote:

> Can one mix Dot 3 and Dot 4 brake fluid in a Vanagon brake system, or is
> that not recommended?
>
> Dave B.
>