Most of the major manufacturers are phasing out oil based paints, opting for low VOC water based, urethane based, or acrylic replacements. This has been occurring since 1979 (California was first), even before the EPA stepped in to back up this play (2009 I believe), and several states have now been having a rolling ban. Retailers and manufacturers can use up what they have, but once it's gone, it's gone. In my travels, when I've needed it, I've found it in quart size easier than gallon size, used mostly for touch up than for overall paint work. Since the old oil based paints get brittle over time, I've used it less and less, opting for a urethane or water based replacement. You do not to prime between the two, but I've done this whenever I've had to touch up the exterior of our home(s), or that of elderly family members.
Bill S.