Originally Posted by Old Fox

The filter appeared dirtier when you were using 90 weight oil since almost all the filtering was happening at or very close to the top surface of the foam where most of the oil was. The viscous 90 weight oil didn't go deeper. If you'd looked closer, most of the dirt would be on the air-box side and very little on the engine side. And if you'd cut up the dirty foam (don't waste a perfectly good filter just for that though ), you would find the core area to be pretty clean. With a less viscous oil the filtering would occur right through the thickness of the foam.

Another effect of a thicker oil is that since most of the filtering is done at the surface, it clogs up sooner and starts acting as a hindrance to the airflow. So in effect, you have a shorter service life of the filter before it needs cleaning again.

Originally Posted by Andante

I would like to raise awareness about one thing. In the pictures for this DIY procedure, we see the topic starter handling kerosene, diesel and petrol with his bare hands. All of these fluids contain substances that can penetrate the skin and cause irritation or worse.

See also MSDS sheets: Kerosene, Diesel, Petrol

So please do not handle any of these fuels with your bare hands!