Hi team - I did use the K&N oil in the aerosol kit, and was also pleasantly surprised to see how much extra gunk and bug guts the compressed air blast got out.
I did give the inside a light coating, but thought the outside needed a hit as well - you may be spot on STroppy in saying only the inside needs spraying - not sure on this one.
As for using other oils.... I wouldn't do that myself, as this oil is designed and formulated by the manufacturer of the air filter and would contain all the ideal science for the filter. I can't see myself ever needing to buy a second can, as you use so little of the contents, and do the job so occasionally - it will take a long time to use up the oil.
Hope this helps.
Cheers, Diesel