Depending on where you live, this may not even be legal. Used motor oil, where I live, must be collected in leak-proof containers and periodically picked up and taken to a depot where it is reprocessed. Using it as a hack outdoor finish is not legal here, and rightly so.
Everything that is in the oil and inside the engine of a car (aromatic hydrocarbons, volatiles, heavy metals) will end up in your local environment over time.
If we were living in a time where good outdoor finishes weren't easily found, or were too expensive, you might see motor oil as a poor last choice. At some point all kinds of petroleum products were used in this manner. For example, it was common for diesel fuel to be put to all sorts of uses when it was cheaper and easier to come by (and the cheaper stuff wasn't dyed red) than alternatives. Many of us are living in houses where the basement was sprayed with petrochemicals as part of the concrete pour. You aren't allowed to do that anymore.
It just makes no sense today. It'll make a terrible finish and to add insult you will have to reapply regularly.
This whole idea is so astonishing in 2021 that I'm wondering if the wood whisperer you know doing this has ever actually used the internet, or even read a booklet on finishing outdoor projects from the last three decades.
Please, just skip this and use one of the good outdoor penetrating finishes and commit to splashing on more coats semi-regularly, or go for a sealing product that'll last years between reapplications.
Modern finishes are so much better than this worst, last choice.