Every engine pushes some oil vapor out of the crankcase, and the PCV system routes it straight back into the intake. On a direct-injection engine there is no fuel wash to keep the intake valves clean, so that vapor bakes onto the valves as carbon. On a turbo diesel it coats the intercooler and piping.

An oil separator intercepts that vapor and drops it into a reservoir you empty at oil-change intervals. Simple part, meaningful protection over 100,000 miles.

Driver side or passenger side?

Many engines have two PCV circuits and take a separator on each side. Kits are sold per side — check which circuit you are addressing, or run both for full coverage.

Maintenance

Check the can at every oil change. How fast it fills depends on engine condition, climate and how hard the truck works; a towing rig in cold weather fills faster than a commuter.