Silicone grease is best for the slide pins. WD40 is too thin and won't last. FYI, caliper rebuild kits can be purchased instead of purchasing new calipers, so long as the piston is not corroded and the piston bore inside the caliper is also in good shape and one feels competent enough to rebuild it. When compressing the piston, usually the fluid will push past the master cylinder cap seal long before it'll push past the caliper piston seal. I use a vacuum brake pump kit to suck the fluid out of the reservoir before compressing the piston, but a turkey baster will also work; just don't use it for food afterwards!