If you’ve spent any amount of time watching car YouTubers who are into pulling abandoned cars out of barns or fields and trying to get them to run and/or drive home, you’ve surely seen someone use diesel oil in an old car. Derek from Vice Grip Garage is famous for this, and he stands by his assertion that Rotella T4 has all the vitamins and minerals an old engine needs to keep it happy and healthy. He’s not wrong. Using diesel oil isn’t a bad idea in older engines, it doesn’t really hurt anything. In fact, in some cases it’s actually beneficial.
We’ve discussed in other posts how older engines with flat tappet cams need more zinc than modern oils usually have to avoid flat-spotting a cam lobe. In addition to that, diesel oil has some other benefits when used in older mills. For example, it’s usually heavier than regular motor oils, and using a thicker oil in a tired-out old V8 from the early 1970s (for example) can help boost compression and make the engine run better. At least for a little while.
Now, I hear what you’re asking. You’re asking if there are any downsides, any hidden dangers to using diesel oil in your classic vehicle that you need to look out for. Eh, kinda, yeah. Let’s talk about it.
The cat(alytic converter) is out of the bag
The first catalytic converters were developed in France at the end of the 19th century, but didn’t gain widespread acceptance on mass produced autos until the mid-1970s. I’m sure most of you are aware of what a cat is and what it does, but for the uninitiated, a catalytic converter is essentially a filter in a vehicle’s exhaust system that uses all kinds of arcane metallurgy and other esoteric tech to remove a percentage of the toxins caused by the combustion process from an engine’s exhaust. While cats are sensitive to any kind of contamination, they’re particularly allergic to zinc. Since all cars consume a little oil as they operate, and that typically comes out of the exhaust, oil with high zinc content has a tendency to plug catalytic converters up real quick.
Diesel oil, as we discussed earlier, has a lot of zinc in it! That’s why some gearheads use it instead of ZDDP additive to protect the innards of their classic car’s engine. The problem is, if your classic car has a catalytic converter — and oh boy do I, as a gearhead of a certain age, hate the fact that some cars with cats are old enough to be considered classics — you run the risk of ruining said cat with a high-zinc oil. Especially if you have any significant blowby. The rise of catalytic converters is part of the reason oil companies got away from putting a bunch of zinc in motor oil in the first place.
If you’re planning on running diesel oil in a classic car equipped with catalytic converters, the best thing you can do is not run diesel oil in a car with catalytic converters. The second best thing you can do, if you insist on the Rotella, is to make sure your engine consumes the bare minimum of oil and keep an eye on your exhaust system.