[RanK2]

BACKGROUND INFO:
Interior walls were painted with an oil based paint many years ago (maybe 30 years ago). Paint is still in good condition, no chalking or peeling.

Oil based top coat paint is pretty much going the way of the dodo bird so planning to switch to water based acrylic paint for the new top coat (same color).

I know that latex paint can be painted on top of oil based primer since I've done that before. I have a gallon of oil based primer so I plan to use that until it is used up, for the interface between the old oil based paint and the new acrylic top coat.

QUESTION:
However, when that gallon of oil based primer is used up, would it be better to buy a water based bonding primer or more oil based primer? IOW, would an oil based primer be more guaranteed to stick to the old oil based paint better than a water based bonding primer? (Water based bonding primer would be easier to clean the brushes and rollers)

Yes, forget oil paints, except for some specialty situations. ..... The most important thing to me, and that you didn't mention, is the 'finish' on the existing oil base wall paint. (And BTW, if you haven't started using the 'oil base' primer, don't). ...... If the existing wall paint is a 'flat oil', I would just paint over it. But that is 'only' if there is no sheen to it, and there are no signs of poor adhesion.

Latex paints have been applied over oil/alkyd base primers for decades, it is just that the primers do not dry with more than a 'egg shell' sheen at most. Your wall paint is no different. And personally, I wouldn't waste money with a primer in between, just use a quality wall paint. If second coat is needed, let it cure first for a week at least.