Something's in the air
If the chassis has an air compressor (air brakes), then a dryer should be specified. FMVSS 136 Phase 1 mandated ESP/ESC starting Aug 2017 for tractors, the system is optional for straight trucks. Beyond this, today’s vehicles have expensive and complex ABS and ATC systems including more components subject to failure. The dryer protects the entire air system including the reservoirs and valves from solids, liquid, vapors, and contaminants. A heater is highly recommended if operating in freezing temps.
We highly recommended pull cords, so that it is relatively easy to see if moisture is present. Pull the cords on the walk around to see if moisture is present and if it is, then it is probably time for a filter change.
Compressors come in all sizes, depending on air consumption, but regardless, they all bypass oil by the piston rings. Standard filters remove some oil, but the lions share is removed via the coalescing filter. It has a special layer of desiccant that is made for this purpose. It does not matter where the air is supplied to the compressor, before or after the turbo, oil will be present in the system.
Given these filters are usually mounted inside the frame rail, they are out of sight, out of mind. Most OTR (Over the Road, 3 axle tractors) require a filter change every 3 years. Things like increased service brake count and size, lift axles, applications like refuse, etc, can speed up this cycle to every year or sooner.
Today’s filter elements spin on/off, so changing is relatively easy. The previous AD-9 filter (over a million sold) also has a coalescing option but takes more time and effort to replace the element, given the cartridge is inside the dome and is bolted in place. Proper maintenance reduces valve replacement and prevents freeze up. One wrecker bill and down time, pays for the filters, many times over.