The best sealant on the market is a 1-part urethane. Second best would be a urethane modified acrylic caulking.
Yes, it is possible to apply oil base over latex paint. Priming with an oil base prime is necessary. Even though this is possible it isn't recommended. Acrylic paints are breathable and stretchy. This causes some long term adhesion problems. Modern acrylic coatings are superior to most alkyd paints when used correctly.