I bought the van and the oil is cleaning up.
Upon getting it home, I put a better oil filter on (Pure One) and
replaced one quart of oil with an additive (Rislone) which further
increases the detergent effect of the oil. I changed the oil after
only 600 miles. I kept a sample of the oil and it looked better than
the first oil change did after only 50 miles.
I have now driven the van about 1500 miles after the last oil change
and the oil looks pretty good. I will change it again at about 3000
miles or so and then get back to a 5000 mile service interval. I have
just passed 60,000 miles and am running the High Mileage oil formula
of Castrol GTX and will continue with the smaller micron rated