"I run a fleet of heavy-duty trucks and have learned much about the importance of keeping engine oil clean. My question is how relevant is intake air cleanliness."
On a volume basis, diesel engines can use more than 20,000 times more air than fuel. Airborne dirt is abrasive to engine components, and therefore efficient air filtration is vital in avoiding premature engine wear.
In particular, turbocharger compressor blades are eroded by airborne dirt. This results in reduced efficiency causing engine power loss, increased fuel consumption and higher exhaust emissions.
Quality air-filtration systems are designed to trap most harmful dirt, but regular attention is required to assure efficient filtration without causing air restriction and intake system leaks.
Periodic oil sample analysis is a useful tool for determining if your air-filtration maintenance practice is doing the job because it will identify silicon, iron and other elements common to airborne dirt and wear debris in the oil.