Actually, the air filter is not too restrictive. The other fight I have with drivers is that they take the 200 psi air hose to the filters.
I have samples where we can keep the soot under 1% in 6000 km, which whoud be fine, but most end up between 6% and >12%. I've got some samples with >600 cSt at 100C from soot.
Some suffer from being abused for 200,000 km with high dirt and soot, causing increased wear of the cams, which of course reduces the amount the valves are open and delays their opening. Because of the abuse they were all rebuild at around 100,000 km, but valve train was not replaced.
I've told them to turn back the pump flow, but some engines hava a timing adjustment as well (Volvo, at least). Was wondering about Toyota.