Ken Lewis wrote
I have heard the 2.1L oil-to-water heat exchanger can not be used
                        with the 1.9L exhaust. Hence making for a hotter
running engine.
Ken and the list
    I have had a 2.1 engine in my 1985 7 passenger van for 3 years it is
currently for sale at this time. I have not seen any signs of the engine
running hot. this van does not have AC so the radiator fan has one speed
an I am running the stock temp tstat and fan switch.
    The the 2.1L oil-to-water heat exchanger (VW calls it an oil cooler)
serves 2 purposes first it brings the oil up to temp faster and second
it keeps the oil from spiking in temp. I think it is more often the case
that its needed to warm the oil faster then it is needed to prevent
extreme oil temp rises
Bob