Janet
In general the so called miracle oil additives are snake oil. Under rare
circumstances there are some additives to be used for specific problems but
no for general use.  Use the right oil and change it when it should be
changed.  As for gasoline a bottle of Techron at oil change (or if having
problems that could be a clogged injector) can't hurt and could help.  On
the bumpers unless you are going to be brush bucking a lot the monster
tubular bumpers are overkill.  I have a set on my Westy but today I'd put on
a set of the Go Westy bumpers.  Very strong, look good, and cheaper than the
big tube type.  If you go for new wheel covers be careful, there are a lot
out there that won't stay on a Vanagon wheel because the hub sticks to far
out.  Get covers with enough clearance in the center.