the lifter has as spring inside to make them push on rod to open valve. also the hole for oil to pass through the lifter is as small the ball in a ink pen. if just a small chunk of dirt get in lifte the holes get cloged. put oil cleaner in engine follow directions. and change oil put more in next time you change oil remember you might have to put less oil because of the cleaner. the problem you get from sticking
lifters the valve is not opening you are not getting the proper air fuel mixture and you could burn a valve .yes you can run for a long time like this BUT----- thicker oil will stop flowing through small holes and take longer to thin out stay with 5/20 or 5/30