I've been using a 1/2" gas valve shutoff upstream of my upstream fuel
filter for several years now to make th upstream and downstream
filter changing a little less of a hassle.  It also makes a decent
cheapy theft deterrent if you park in nasty neighborhoods.

Last night I pulled the upstream filter and the downstream filter for
annual replacement and decided to see what they had caught in a year's time.

The downstream filter was made in Poland and I think I bought it at
an AutoZone or Advanced Auto, so I doubt that it was a Bosch.  It had
an aluminum housing as opposed to steel.  It had it's share of dark
colored stuff, but no chunks of rusty gas tank or anything like that
further supporting my theory that an upstream filter  is the best
protection you can give your fuel pump other than keeping from
running the tank dry trying to make the next gas station.

Since my 90 GL: didn't have an upstream filter in it to begin with, I
put a metal in line filter from WIX in there that had male 1/2" barbs
on each end.  Current part number is 33248. There's a 5 in front of
those 5 numbers, but the local parts guy told me that the first
number simply denotes where the filter was made. They make a plastic
one too in the same configuration, but I don't have that number
handy. Paid like $11 for the metal one and the replacement Bosch
filter for the downstream side is likely the spare I got from Bus
Depot some time ago.

Cut the Wix apart first as my 2 wheeled pipe cutter jaws were just
slightly too small to open it up that way.  I unfolded the filter
media and to my surprise there were no rusty chunks of gas tank in
there.  What was curious though, was all the dried grit stuck to the
circumference of the inner metal walls.  Sure glad all that didn't go
through my fuel pump.

Those contemplating doing this mod, there is plenty of horizontal
room for the valve and the Wix filter between the tank and the pump,
but I might need to research just how important it is to have
unrestricted flow from the tank to the pump as the valve body opening
diameter might not be equal to the diamter of the hose, and as the
upstream filter clogs up, that could I guess cause stress to the
pump.  The van's been getting like 5k a year drive time, so longer
driving might need more frequent filter changing.

YMMV

DM&FS