I am new to this forum so a bit behind with this topic. Just thought I would add my 2c's worth for posterity.
I have a Greco Ultra 400 which is now over 20 years old. I bought it in Oz and often would switch from water base to oil including clear polyurethane. As long as you clean the filters out it was fine. When the job was finished I would clean thoroughly with water and then pump some heating oil in for storage.
I haven't had a lot of use for it in the UK since we came back but I do find that most paints in the UK are not designed for spraying, to much fall out for my liking, especially with oil base paints.
I used it once (here in the UK) on three new builds, the guys I was working with were gobsmacked at how fast it was but didn't like it as they thinking was I was taking work off them. A little smalled minded for me. If the OP comes back I would be interested in what reaction he gets here in the UK.
Edd