WD=40 isn't a great lube, but IS an oil, and it's a great cleaner. And if you re-apply often, keeping your chain clean has more benefit than using a slipperier oil. However if you use it and don't re-apply often enough, the lighter oils evaporate out and leave a gummy residue. Motor oil is a better lube, but messier. Most bike-specific chain lubes try to strike a compromise between lubing well and making your chain an oily mess. For my applications, "cleanliness is next to godliness" because the chain runs less than an inch from my knee; so I use a wax-based lube. I still get smudges but they're not oily smeary messes.