There are many reasons why some diesel vehicles, most notably some vehicles of German manufacture are oil guzzlers, but suffice to say that the principal reason many high-mileage diesel vehicles use excessive amounts of oil is because the turbo charger shaft seals are worn, damaged, or defective.
Essentially, highly pressurised engine oil is blown into the inlet tract past worn turbocharger shaft seals, and the high flow rate of the intake air then transports the oil mist into the intercooler (if fitted) where it collects on the inner walls of the tubes. However, in some cases, there is so much oil present in the inlet tract that much of it is carried directly into the cylinders, where it only partially combusts. The added load of unburned hydrocarbons then moves through the exhaust system to be deposited onto the substrates (cores) of both the catalytic converter(s) and the DPF. In time, the substrates then become clogged with oil residue, at which point both stop working altogether.
This condition is hugely expensive to repair, since both the catalytic converter and the DPF must be replaced with OEM, or OEM-equivalent parts to restore the emission control system’s efficiency. Note though that although there are so-called “refurbished” or “rebuilt” units available, these are, more often than not ineffective, or when they do work, they do so only for a few hundred kilometres, at best.
Bear in mind though that simply replacing the catalytic converter(s) and the DPF will not cure the underlying mechanical issues that cause excessive oil consumption, but depending on both the vehicle and the exact nature of the problem, fixing the underlying mechanical problems might run into many thousands of dollars. However, there is a cost effective short-term solution to excessive oil consumption, the fitment of a oil catch can.
As the name suggests, an oil catch can is a canister that is installed, or spliced into the intake system between the positive crankcase ventilation valve and the intake manifold, and its function is to “catch” excessive amounts of oil that might be present in the intake system before it can enter the cylinders.