soft plugs to me, are the cupped or flat core plugs found in the sides, or the front and back, of engine blocks and cylinder heads.
i usually use permatex #1 after a good cleaning of the block and plug. i think these are also called "welch" plugs by some ?
as to the threaded taper pipe plugs at the end of the lifter oil galley, loctite #567 white teflon paste works great ! that stuff is almost "anything" proof, and can be used with any material or combination of materials. stainless, brass, iron, you name it. i don't use anything but this on taper fittings. it's also good for high temperatures and pressures.