the tail khana shuru is 95% a case of bad valve seals and bad injection timing, valve seals are obvious causes but the bad injection timing causes the rings not to seal in the cylinders which then causes oil consumption. To make this problem speed up to engine death - simply take out the thermostat valve, it surely will destroy the engine very very soon. Its quite common in Pakistan because almost no public commercial repair shops have proper diesel engine repair tools. All the tools they know are
paana
taami
goti
rechutt.
There are thousands of cases of diesel engines going strong for more than 1,000,000 kms on original engine with just upkeep maintenance - this does not only mean oil change, it means proper checking and adjusting of injection system, air intake system and cooling system.
Honestly the only time I fiddled with the air filter was when I was doing the general checkup maintenance - even in Karachi that was 1 month apart. a slight tapping of the filter and some compressed air would clean it, I changed it every 12 months. (But I drove a VW 1.6 diesel - it takes a gigantic oil filter and a very tight packed air filter like the fiat UNO)
The original toyota diesel air filter is very durable and is good for about 10,000 kms in severe duty. Make sure the entire airbox is sealed and your valve clearances are good - otherwise the engine backwashes air from the filter housing.
btw this is my E300D filter after a year of use, the car has travelled about 14,000 miles on this filter. Costs 11 dollars.
a similar filter was used in my VW Golf.
