Clear Oil Pan Shows How Much Dirty Oil Remains In A Duramax Diesel After A Change, But Does It Actually Matter?

The internet is full of aftermarket companies trying to sell you products to solve your automotive issues. They’ll tell you there’s something wrong with your car, and they’ve got the product to fix it. Banks Power likes to go a little bit further, though. They don’t just tell you what’s wrong—they’ll show you! It’s then up to you to decide on what you’re seeing.

Previously, Banks Power did a great job explaining what’s wrong with the Cummins 6.7-liter diesel. They showed us how a single nut can get ingested into the engine, causing terminal damage in many cases. Their video showed us, with specific examples, how corroded or weak grid heater components were causing all the trouble.

Earlier this week, Banks Power returned to YouTube to show us an issue with the 6.6-liter Duramax L5P engine, as used in the Chevy Silverado HD and the GMC Sierra HD. We’re told that GM’s oil pan design is trapping dirty oil in the engine, even after an oil change! The video presents this as an “engine killer,” which seemed pretty drastic to me. Let’s examine what’s going on.

Old Oil

Banks Power develops a wide range of accessories. In regards to GM’s Duramax-powered trucks, it has developed a range of upgrade parts designed for heavy-duty users. To this end, the company produces  upgraded differential covers, transmissions pans, and oil pans. They’re designed to help keep the vehicle’s vital fluids within an acceptable temperature range during sustained high-load situations like towing, achieving this through the use of cooling fins and greater fluid capacities.

However, it was during development of the Duramax oil pan that the team found something curious. “There’s a quart of dirty oil trapped inside your engine,” says Erik Reider, special projects lead at Banks Power. “No matter how many times you change your oil, it’s not coming out,” he explains.

The cause of the problem is relatively simple. When GM designed its stamped steel oil pan, it had to make provision for the sump plug. Since the stamped steel is very thin, it doesn’t have enough thickness for the plug to thread into. To get around this, GM used a weld nut. It’s basically a little threaded nut that attaches to the sheet metal and provides enough thread engagement for the sump plug to work properly.

The thing is, the weld nut sits proud inside the sheet metal oil pan, by around 3/4 of an inch or so. Thus, it creates a lip, so that the the oil pan outlet is not actually at the lowest point of the sump. This means that when draining the oil, some oil pools around that lip without fully draining out of the engine.

It seems wild that the stock pan would work like this, which Reider readily admits. “You’re skeptical,” he says. “There’s no way that GM would allow this dirty oil to be trapped inside the pan for the life of the vehicle.” However, he has a way to prove it—Banks Power 3D scanned a stock oil pan and made a similar part in clear plastic. This would allow the engine to be filled with oil, and then drained, to demonstrate roughly how much oil remains in the engine after draining it.

[Ed Note: This type of oil pan drain plug design is pretty standard stuff.

Above is a look at my Pontiac Aztek’s new (aftermarket) transmission oil pan. -DT]

Between the oil pan and the oil pickup tube, around one quart (0.94 liters) — or 10% — of oil remains trapped in the engine during an oil change. That’s quite a bit of old oil to be left behind. In particular, because of the weld nut, the oil level remains high enough to stop air reaching the oil pickup, which leaves some oil trapped in there as well.

“I think disturbing is the right word for this,” says Reider, noting the amount of oil that was still left in the pan. “This means that if you have the stock pan, you’re stuck with a quart of dirty oil, literally forever,” he says. “It’s got metal debris, soot, it’s been oxidized, aerated, none of this is good for your engine.”

You’d think that any oil remaining in the pan is effectively diluted by the ten quarts of new oil that would be put in during an oil change. However, this does mean you may not be draining out stuff like metal particles, which instead may stay behind in the sump. That’s not ideal. At the same time, if you’ve got a significant quantity of sizable metal particles in your oil, you’ve probably got other problems, anyway.

It’s worth noting that Banks Power made this video for a reason—it’s effectively an advertisement for their own oil pan for the Duramax engine. Their design places the drain plug at the absolute bottom point of the oil pan. With no appreciable lip, it allows virtually all the oil to drain out of the engine during an oil change. It’s also got a magnetic oil plug for attracting any ferrous metal particles that may end up in the oil. The design also allows the oil level to drop beneath the level of the oil pickup when fully drained, allowing that part of the oil system to drain out, too.

The redesigned oil pan also has a greater oil capacity versus stock for increased thermal mass. With two extra quarts of oil in the engine, it takes longer for the oil to heat up to critical levels. “It holds two more quarts of oil over stock, which slows heat saturation thanks to the increased thermal mass,” says Reider.

Of course, that’s not all that matters, though. “Once all of that oil is hot, it’s hot,” explains Reider. “What really matters is how you reject that heat. ” To that end, the pan also features large fins to improve convective cooling—it’s designed just like a heatsink you might find inside your computer.  The idea is to help keep oil temperatures lower during heavy-duty running, such as when towing uphill. By keeping oil temps below set thresholds, the Duramax ECU will allow the engine to keep running at maximum output. Banks Power isn’t the only game in town, either; as covered by Diesel World, aftermarket manufacturer PPE has an oil pan of its own designed to similar ends.

Is this a problem that requires aftermarket parts to solve? I’m not necessarily convinced. As covered by repair shop ProSource Diesel, the L5P has a pretty good reputation, mechanically speaking. It’s been in production since 2017 and hasn’t had any major recalls for oil system issues or the like. Ultimately, if this “trapped” oil were a big problem, we’d see lots of evidence by now—owners complaining of sludged-up engines, or common problems with bearings and valve trains and the like. Ultimately, Banks started developing this part to improve cooling—solving the oil drain issue was just a nice bonus that makes the part a lot more attractive.

From my professional standpoint as an engineer, two things are true. Yes, the stock oil pan design does leave some oil trapped in the sump after an oil change. At the same time, this isn’t necessarily a big deal. Obviously, it’s ideal to drain all the oil, particularly if there is any particulate matter in it. But for a properly-running, undamaged engine… leaving a quart of older oil in the sump isn’t going to particularly hurt anything. After all, the engine was running just fine on that oil immediately before the change! It’s also a little disingenuous to say that the oil is “trapped forever.” You’re not left with one single bad quart for the life of the engine. That quart of oil is mixed with the new stuff as soon as the oil pump runs the first time. It’s just that on each oil change, some of your current oil is left behind. And as for particulate matter, capturing that is what the oil pickup screen and especially the oil filter are for.

I’m not the only one that thinks this way. Head over to the r/Duramax page over on Reddit, and many owners and techs feel the same way. “This video is a perfect example of scare tactics,” says Kennel_King. “If that quart of oil being in there was that big of a deal, these engines would be failing left and right.” Others note that this story has played out with previous generations of Duramax engines before, too, which similarly trapped some oil in the sump. “My ’01 LB7 is at 1.48 million miles and I’m about to tow across country,” says tuskenDMAXer. “Still got the original oil pan for the original engine and rear differential cover for the original gears and bearings, even though Banks says they’re no good.”

If you’re changing your oil regularly as you should be, that last bit of oil will be diluted by the new stuff each time. The fresh oil will do its job, and it won’t be unduly harmed by the quart or so remaining from before. You can buy an aftermarket oil pan if you really want one, but I wouldn’t unduly worry about it.

Image credits: Banks Power via YouTube screenshot