Engine oil works OK, if it is cleanish. Obviously, it is necessary to limit the quantity to prevent overspill.
If I was going to use oil, I would use used gear oil, or even chain oil, which is quite sticky.
Fish oil is the best to combat rusting, it has amazing "wick in" penetration qualities, as it dries forms a lacquer type skin but remains wet beneath it sticks like sh1t to the blanket.
Back in the mid 60's I owned a rusty Fiat 1800, (rusted, the only kind of Fiat they make!) which had rust coming through any place where there were seams in the metal, I sprayed copious quantities of fish oil about everywhere there was rust, that managed to preserve the metal and arrest the rust for years after, I sold it in '73 rust no worse than when purchased. Just that it smelled a bit of fish and the local cats loved to sleep on my car.
I use deodorised fish oil now on my beloved Disco D1 wherever it looks like getting a rust area and have sprayed it about in the chassis, not not the full treatment yet, but it's a lot like beetroot juice, a little goes a long way and it gets everywhere.
 

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