Mark,
Look at the drivers side of the pan.
It has a compression fitting type nut on the end of the tube.
Buy 4 quarts.
I believe it takes 3.5 quarts.
I'll look at my owners manual tomorrow and report back then, to be
absolutley sure.
In your own best behalf, ( and the Van's) drop the pan and clean and
inspect the screen.

Smarter move than just draining the fluid.

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Terry
74 Campmobile (Clementine)
85 GL
86 BMW 325 ES