OK, thanks for the additional info. My brass fittings that *don't* interchange simply don't fit together at all, frustrating when I run across one like that in the middle of a job but I'm gradually getting them sorted out.
Most of my washers were replaced long ago/many times...not sure what I'm using in most of my fittings now.
While shopping at Lee Valley, check this sprayer out, I love it for undercarriages and wheelwells:
Lee Valley Tools Under-Leaf Sprayer