Not sure if it’s recommended or not though, it just seems practical and probably using a negligible amount of gas just to warm up…
basically you heat it up, but you don't finish, and you never planned to

That is bad behavior on many levels. You know you will get on the highway and do a long trip using the engine, fine. Otherwise is bad

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So when a petrol engine warms up, some fuel will pass in the oil. This is normal and fine. Once the engine is up to temperature, this will stop, and the existing fuel in the oil will evaporate as you go, gets separated from the oil, all will be fine. If you just warm it up but you don't follow trough and run the engine for a significant amount of time, that fuel will stay there. And then you do it again and again until the oil becomes so contaminated that is does not lubricate anymore. And this bad.
On a normal petrol car, not a big issue. Eventually for sure the engine will run long enough, you have to, if you want to move. In a plug in hybrid, you do this a few times( warm the engine up with no or little use after ) ,and go electric only.. - problem..
Luckily the engine management monitors this kind of bad behavior and catastrophic failures will be prevented.. but there will be some trouble.. see the FORM error..