Khartoum is a fairly modern city; I hope you have access to a head and base gasket and engine refurbishing services. That amount of oil consumption is alarming if it is not an external leak.
These air filters rely on oil in the filter to catch dust. Without sufficient oil worked into them they can not filter properly. It doesn't take much sand to dust a piston, and that destroys the rings' ability to control the oil on the cylinder wall. Pistons in engines that burn oil are usually very clean on the top of the piston. The burning oil tends to clean carbon off, so there would not be a carbon build up to cause pre-ignition and pinging or knocking. A pinging or knocking sound on a dusted piston is likely to be piston slap, as the piston is not running up and down smoothly, but rather rocking back and forth in the cylinder. The skirt is hitting the cylinder every time it reverses direction.
Unfortunately, the only way to check is to remove the head and cylinder to get a look at the piston.
Here is what a dusted piston looks like.
This one was pretty bad, burning a liter in several hundred kilometers. It had a loud piston slap, too.
The only fix, obviously, is a new piston, rings, and cylinder bore.
The other problem is the head. If it was run low on oil, to the point where there was no oil for the pick-up tube to stay submerged, and the head was starved of oil the first thing to go is the right-hand exhaust cam journal. Checking that will tell you if the head is ruined.
How much further do you have to go on your trip, what resources do you have for repairs and how good is your mechanic?