a.) It's good, however, it doesn't last that long, and then you'll just have to relube it.
b.) I find that it doesn't work that well in 3x3's (as in no effect), but it made my Shengshou 6x6 much better. It really improves the Wittwo 2x2 too.
c.) I do recommend it; it's nice and cheap and makes big cubes a lot better.
d.) Just a few drops around the core, and on the pieces. Then scramble the cube you lubed for like 10 seconds and let it sit for a few minutes. It should be good then.