What you see is sludge, a combination of oil and condens.
My L3 650 had the same problem and I have tried a lot of things to solve this. When the outside temperature is above 15 degrees Celcius the sludge will disappear.
A few years ago I made a topic off this with some pictures in the Dutch V-Strom forum.
https://forum.v-strom.nl/viewtopic.php?f=5&t=13429&p=195878&hilit=luchtfilter#p195878The main problem is that the oil temperature is not high enough when it is cold outside.
Via the crankcase vent, condens collects in the airbox, and flows back in the oil.
Because of the low temperature of the oil, the condens in the oil will not evaporate complete and will become sludge.
The airbox of my Wee has a drainhose, with which I can drain condensation from the airbox. The problem is for most of the time solved now.