The dirty side is where the touches the filter first. On a tube and conical filter, the outside is the dirty side.
Use a brush to get any debris off of the filter. This will be leaves, dust, dirt, mud etc.
Create a bowl full of hot soapy water, not too hot that you can't put your hands in it. We only want to clean it, not
melt the foam and soften the glue that laminates the layers together. Submerge the foam filter into the water and
give it a good massage to get the soapy water into the pores to break up the contaminants being held inside.
With a tube and conical filter, run clean water from the inside, this flushes out all of the contaminants.
On a panel filter, run the clean water from the clean side through to the dirty side.
The reason for this is the pores of the filter get smaller as they get towards the clean side so it's easier
to flush the contaminants through the larger pores as it's less likely to clog the filter. NEVER use a pressure
washer and NEVER blow a foam filter with a high pressure air line. This high water or air pressure will
damage the structure of the foam causing it to be less efficient and filtering.
The filter now needs to dry. NEVER try to speed this up with a heat gun or a hair dryer. Shake the filter to
get as much of the water out as you can. You can even try putting it in a pillow case and spinning it around
in that to force the water out. Then either place it on a hot radiator or a hot window sill to dry out fully.
Don't let the foam filter dry out too slowly as it can start to grow mould which will clog the filter.
Now the most important part, reoiling the foam filter.
This is a step that many people just don't bother with. A dry foam filter will filter out twigs, leaves
and the odd squirrel but when it comes to dust, it'll pass straight through and into the engine.
The oil bonds with the foam creating a sticky surface. It's important to fully oil the filter all the way through.
As the air passes through the foam it is forced to change direction over and over. When air changes direction,
it hits the foam structure and the oiled structure of the foam grabs hold of the dust and keeps it held in place.
Once oiled, refit the foam air filter and you are done for another year.
This seems to be a topic of confusion for many people but it's easy when you know the facts.
People claim that any oil can be used to oil a filter. This isn't true.
People also claim that foam filter oil and cotton gauze (K&N etc) filter oil are the same. Also not true.
Foam filters have very open pores compared to cotton gauze filters so you need a thicker oil.
Putting a thicker oil on a cotton gauze filter will clog it and reduce its efficiency.
Bottles of foam filter oil are generally quite expensive but there are cheap alternatives.
Any thick motor oil is sufficient. You can buy chainsaw oil from many DIY stores.
The alternative is Shell's Advanced Filter oil. It's very thin but contains an adhesive that makes it very sticky once
it's applied. If you use this stuff (available from eBay), don't get it on your skin, it's waterproof and hard to get off.
If you are using the Shell spray, don some waterproof gloves and spray half of the can onto the
filter. Inside and out. Massage it into the filter and then let it sit for 10 minutes before fitting.
If you are using thick oil then pour 100ml into a bag, place the filter in and massage the oil into the filter through
the bag. Remove the filter and give it a flick (somewhere you don't mind getting covered in oil) and let it sit for
30 minutes. This allows any excess to drain out. You'll be able to deduce the approximate amount of oil the filter
actually held for next time you clean and oil it. This will save time waiting for excess oil to drain out.