I have this problem(?) with my 84 GL....sometimes it's one side of the motor, 
sometimes the other.  My mechanic said he has the same problem with his 91 (and 
with his older models).  What he suggested is when changing the oil, add a 
quart of solvent to the oil, start the sucker up and let it run for ten 
minutes, then shut it down and do a regular oil change.

1. This solvent is a Marvel product...not their "Mystery Oil" but something 
else.

2. The oil comes out like water after this treatment...so don't rev the motor 
or drive the van with this stuff in.

3. Of course, you need to change the filter when you do this.

I've done this three times now, and I've found it makes a big difference in 
smoothing out the motor and improving mileage.  The only question I really have 
is why so much crud builds up in only 3000 miles.  If anyone wants the name of 
this stuff, I'll try to dig it out.

Peter Eaton
84 GL "The Land Ark"