Two possibilities jump to mind:  Mobil 1 Synthetic ATF, which Mobil says meets multiple specifications, including Dexron III and (better for our purposes) Dexron IIIH (the 'hot weather/heavy loads' version of Dexron III that was developed for Allison), and AC/Delco's 'fully synthetic' Dexron III.  Both are red.

Mobil 1 claims their synthetic ATF is compatible for top-ups with all Dexron III fluids.  See:
http://www.mobil.com/USA-English/Lubes/PDS/GLXXENPVLMOMobil_1_Synthetic_ATF.aspx

James
Ottawa, ON
'91 Westy Weekender AT

> Date: Sat, 2 Jan 2016 18:17:02 -0500
> From: ecaron1@COMCAST.NET
> Subject: Auto Synthetic fluid recommendation needed
> To: vanagon@GERRY.VANAGON.COM
> 
> Hello helpful listers,
> 
>         Two years ago I had a local garage change my auto transmission fluid to synthetic.  I can�t find a record of the type of fluid used.  I need to add some fluid as the indicator is showing the level now just below the lower line.  
>         I had the fluid checked with engine hot and running.  I�m also told there is a three or four inch red spot on the tarp where I park the van.  so I assume the fluid is red.  
> 
> Anyway.  As the fluid is hopefully synthetic what is the proper synthetic automatic transmission fluid to add?  Parts counter folks have not seemed to have a good recommendation.
> 
> Also with the fluid just below the low line how much should I add to try to get it to the middle?  This will be tricky as I believe this is added through the dip stick opening.
> 
> So most important what Synthetic fluid should I add?
> 
> second how much for the first attempt.
> 
> Any other tips or info is appreciated.
> 
> Thanks folks 
> 
> Eric Caron 
> 85 GL Auto Westfalia