904 lockup fluid question

[ManuD]

Hey folks
I just got a 904 lockup to replace my dying one on my 1982 half ton campervan (B150). I am wondering what type of fluid I should use for the use I am going to make of it. (I know, there are lots of threads on it)
Anyway, I am towing a heavy van on long distances sometimes in lots of hills (British Columbia). My dipstick says to use Dexron 3, my Haynes books says to use ATF+4 when it<s lockup.
I don<t know what fluid there is in the tranny right now. It is still bolted in another van that I got. The thing shifts smoothly and will softly knocks once in a while going from 2nd to 3rd. The fluid also smells burnt but has a nice colour.
What do you guys think?

 

[67 coronet]

the +4 is pretty much a synthetic version of dex 3.couple more addatives,but about the same.synthetics tend to be more stable,so i would run that.dont do the type f change over.it will give you harder shifts,which is not the best for hvy towing.