WD40 won't last that long. ATF is a cleanser and very good at freeing stuck rings. I personally use a little oil. Put a couple of capfulls down the spark plug holes and just rotate the motor over a couple times to get a nice coat on the cylinder walls to prevent rust. If the motor is being stored in a humiid environment like a shed or garage it would be a good idea to get some oil on the valve springs from time to time.