Husqvarna 356BTX-series Operator's Manual page 18

Clean the filter every 40 hours, or more regularly if
conditions are exceptionally dusty.
Avoid contact with hot surfaces on muffler, cylinder etc.
Contact can result in burns to the skin.
Cleaning the air filter
Dismantle the outer cover with the help of the four
fasteners holding the outer cover.
Loosen the two fasteners holding the air filter cowling and
remove the filter. Wash the filter clean in warm, soapy
water.
18 – English
MAINTENANCE
The air filter is fitted in the air filter cowling.
Ensure that the filter is soaked in oil before refitting it.
An air filter that has been in use for a long time cannot be
cleaned completely. The filter must therefore be replaced
with a new one at regular intervals. A damaged air filter
must always be replaced.
Oiling the air filter
Always use HUSQVARNA filter oil, art. no. 531 00 60-76.
The filter oil contains a solvent to make it spread evenly
through the filter. You should therefore avoid skin contact.
Put the filter in a plastic bag and pour the filter oil over it.
Knead the plastic bag to distribute the oil. Squeeze the
excess oil out of the filter inside the plastic bag and pour
off the excess before fitting the filter to the machine. Never
use common engine oil. This would drain through the filter
quite quickly and collect in the bottom.