Yes, you can use a plastic-safe "wet" lubricant like that to lube your keys, but it will only lube the slider and possibly the tactile "legs", not the spring. If you use a relatively thin lube it should go in easily and dissolve / wash away any gunk that could be making them stiffer, although I really can't think what could cause Browns to become stiffer with use, as I've said before.
Worth a try.