We all know that if you apply too much oil when reoiling a K&N air filter you might get oil migrating onto the fine wire in the MAF and screw up the MAF, requiring the MAF to be cleaned. Below are instructions for determining how much oil to put on your particular K&N air filter (filters vary in size, depending on the vehicle make, model, year, engine, etc., and they therefore vary in how much oil you should apply).
The K&N home page says the following for question 27 of the FAQ's:
"27. How much oil should I use to re-oil my filter after it is cleaned?
To determine the amount of oil needed for your filter, enter your filter's K&N part number in the Search by Part box on our Application Search Page. If you don't know the part number you can look it up on the same page. Be careful not to over oil your filter."
If you go to the K&N home page, which is: http://www.knfilters.com/
and find the part number for your vehicle's air filter, and click on the part number, you get a product specs page. That page will tell you how much oil to apply to the filter when reoiling. For example, below is the K&N home page site product specs page for the air filter for my 2003 3.0 liter V6 Ranger. Notice that under "Product Specifications" it states "Filter Re-Oiling Amount: 1.03 oz."
Below is the K&N home page subpage for cleaning accessories:
Note that for oil you can get either a 6.5 ounce spray can or a 12 ounce spray can or an 8 ounce squeeze bottle.