After cleaning my filters, whats the best oil to spray on them. Would W D 40 work ?

In a word NO!! What type/ brand of filter are using will dictate what filter oil to use.

K & N filters. would a light oil spray work ?

Depends a lot on what you're running, pavement or dirt. K&N has their own filter "oil", I like Klotz, for the paper filters, you really need to be careful, too heavy a media, and you're too rich, too light, and you've dusted a motor, so you want to take that into consideration, I'd rather use a little less, and clean more often, than use too heavy, or too light, and mess up the air flow. JMO! Bob!

Marvel Mystery Oil

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All the auto parts chain stores sell a K&N filter charger kit that comes with a bottle of K&N cleaning solution and a spray can of K&N oil.....on a dirt car you will use up all the cleaner way before you use up all the oil in the package. No reason to use anything else when the stuff made for it is so easy to get ahold of.