I worked on an '85 van a little while ago that had a bad fuel pressure
regulator.    It ran very rich after replacing the regulator also.      It
turns out that a lot of gas has been pumped into the engine due to the high
fuel pressure and gotten down into the crankcase and diluted the oil.
The breather fumes were gas laden going into the air intake and making the
mixture rich.
I changed the oil and that was the end of the problem.

good luck,   Steve Cotsford