WD-40 is <u>not</u> an acceptable air filter oil IMO... And I would advise against it.

Use oil designed for 2/4 stroke air filters, such as motor cycle/quad air filter oil. You want the air filter oil to be thick and sticky. Thick to stay put in the filter and not evaporate and sticky to catch the dirt/debris thatis drawn through the filter.

WD-40 is neither thick or sticky.