Years ago I read never to use WD-40 on a gun. Trust me this is still on the subject, so read on. It seems that when you spray WD-40, water in the air may condence out and hit the steel of the gun along with the WD-40. Have you noticed how cold the WD-40 gets as it leaves the nozzle? You may find rust on the gun and wonder why. It sounds reasonable to me. I put nothing in my engines and have never noticed a negative. I have 4 OS.46FX engines, 8 years old. Sure wish at least one would wear out so I can justify a new engine. A rough calculation shows about 30-40 gallons thru each one.