BTR Hydratrak fluid capacity

[Flyinghorse]

I just changed the fluid in my BTR Hydratrak differential.
   The chassis books says 3.12 litres capacity. I bought 4 litres of expensive diff oil and found it only takes 1.5 litres.
   
   I measured what I drained out as 1.5 litres.  Thinking about it 3.12 litres is a lot of diff oil....
   
   I wonder what the other chassis books state as BTR diff oil capacity?
   
   Graham