"I’ve been hearing different things from different shops. One guy tells me every oil change, another guy says once a month regardless of miles. I’m running a Cummins X15 and I don't want to mess up my injectors. What’s the rule of thumb for an owner-operator who's actually putting in the miles?"
Mechanic Answer:
"In the world of trucking, your fuel filters are the only thing standing between you and a catastrophic 'engine derate' on the side of the road."
While the manual might give you a high mileage interval, real-world conditions (like getting a bad batch of fuel at a high-volume stop) change the math. If you wait until the light on the dash comes on, you've already put stress on your high-pressure fuel pump.
The Owner-Operator Standard:
The 15k-25k Rule: Most pros swap the fuel/water separator every 15,000 to 25,000 miles. Many owner-operators find it easiest to just do it every single oil change. It’s cheap insurance.
The Sight Glass Test: If your separator has a clear bowl at the bottom, check it during every pre-trip. If you see a distinct line of water or dark sediment at the bottom, drain it immediately using the valve. You don't necessarily need a new filter yet, but you need that water out.
Winter Strategy: In freezing temps, water in that separator turns into ice crystals that can plug the filter and starve the engine. If it's below 32°F, you should be even more vigilant about draining the water daily.
Pro Tip: When you swap the filter, do not "pre-fill" the new filter with fuel from a gas can. That fuel is unfiltered and can send a tiny speck of dirt straight into your injectors. Install it dry and use the hand primer pump on the engine block to get the air out. Your injectors will thank you.