All you have to do to determine the type of existing paint is take a cloth or paper towel, dab it in some rubbing alchol or goof-off, then rub it on the paint a few seconds. If the paint starts to soften and transfer onto the cloth, you have latex. If unaffected, you have oil-based. Sounds like you need to prime it. Zinsser makes a product I recently found out about called "Peel Stop". Haven't used it, but I'm a big fan of Zinsser. You have to go to a paint store to get this stuff. It may help with your latex over oil problem. In the past, I've dealt with latex over oil problems by disturbing the existing paint as little as possible, then priming with 123, painting, and praying. This has worked quite well for me, but your mileage may vary depending on how well the existing latex is gripping. One other note, be careful in an old home like that. It's bound to have lead-based paint somewhere.