Air filter, one of the worst offenders.
In a diesel/gas engine it will ruin a turbo and cause it to go through oil like a crazy too. Air filter.. yup...Under 100.00 usd on a HD diesel, 20-45.00 usd for a gas rig..
Second bad fuel filters (low fuel pressure)... Can cause the newer injectors to take a dump... they will leak (drip) or over fuel, and cause cylinder wash, again hi oil consumption, and a lot of premature ware, no lubrication...
Regards