Okay,  I just filled my 091 4-speed tranny with Redline MT90.   The tranny is out of the bus and was totally empty.
I carefully leveled it and pumped in 3.7 US quarts, but it did not bring it up to the bottom of the fill hole.  So, I
checked Bentley again and it says in a footnote that the capacities given are for a RE-fill because normally
1.5 quarts will remain in the tranny even after draining it (assuming, I think, that tranny is left in the vehicle
and that the vehicle is level).

So, I pumped more MT-90 into my tranny until it was just at the very bottom edge of the fill hole.  It took  right
at 4.7 US quarts.  Interestingly enough, if Bentley's numbers are correct an empty 091 ought to hold 5.2 US quarts.
Go figure.

Now, my official recommendation (caveat; I'm a newbie).  If you have an 091 4-speed tranny, whether it
is in or out of the bus, fill it to the bottom of the fill hole and call it good.  If you have an 091/1 tranny,  fill it to within
15mm of the bottom of the fill hole.  Sounds a lot like 5/8 of an inch - sort of like Karl said all along.

Now if I can just get this thing put back up in the bus...

Warren Lail
88 Westy "Billy Bones"

"Then followed a battle of looks between them, but the captain soon knuckled under, put up his weapon,
and resumed his seat, grumbling like a beaten dog."