"Positive" valve stem seals clamp onto the top of the valve guide, and the valve stem slides through them. Umbrella seals hold onto the valve stem, and ride up and down over the guide to deflect oil away from the top of the guide. Positive seals require a machined surface at the top of the guide for the seal to clamp onto that's concentric with the valve stem. The only time I use them is on a race engine with dual valve springs that has no room for an umbrella seal inside the spring. Positive seals also require extra clearance between the valve stem and the guide, or the valve stems will sieze up in the guide.
Jerry
