Depends on what it is ...have used WD-40 for some places where a thin oil/fluid is better able to penetrate (e.g. balls/sockets on 3pt linkage),but for large surfaces I've started using more products with lanolin as either part or all of the product's composition.
Usually spray down the working parts of my moldboard plow with a lanolin based product when I'm done plowing for the year. Just due it to keep the plow from getting really rusty from the winter rains here in northern Alabama.
A couple good videos from youtube:
Do Penetrating Oils Block Rust? Let's find out!
Best Car Undercoating? Let's find out! Is Flex Seal the Best Rust & Salt Protection?
(always enjoy Project Farm's videos)
How to Stop Rust - What is the best? Rust prevention experiment
...the penetrating oil one was the one that caused me to start using fluid film and look more at lanolin-based products. My take away from watching all of these though was also that no matter how good the product rust will eventually form (to some degree).....