This is not the reccommended method, but seems to work for me,
Canola oil, $2 a litre from the supermarket.
pour a little into a plastic bag,rub it through the bag, put the filter in the bag and roll it about in the bag squeezing it into the filter both inside and out.Not to much as it goes everywhere, dab the filter with a rag to get off the excess.I only do this the day before I ride and change it straight after the ride. Does not seem to show signs of dust penetrationing through on the inside of the filter even with bull dust all over the outside it stays clean.
I wash the filters in warm( not hot) water with laundry powder and a bit of normal washing up fluid also.
I have tried a expensive filter oil that washes out in soapy water, it seemed to make the filter go all limp and out of shape once it was washed, so I stopped using it.
regards,