Hey mate. Can't help you where to fit it as I haven't got a 550, but as for whether it should be before or after the factory filter depends on the micron rating. I have a 30 micron stanadyne in my d22, which is before the factory filter, acting as a pre filter and water separator. The factory fuel filter should be around the 2 - 5 micron range, so anything above this would have to be set up as a pre filter. The lower the micron number, the finer the filtration. So it would be pointless having a large micron filter after a smaller one. On the same token, running 2 filters of the same or similar micron levels is also rather pointless and will cause too much restriction to the fuel system when the first one blocks up to be of any benefit. If your donaldson is the same micron level as the factory filter, you might consider ditching the factory one and just running the donaldson.
