First, it is known that nails are more flexible when the water content is higher. And when you’re nails are more flexible they’re less likely to chip and break. But surprisingly, oil alone does not have much effect on nails. Researchers treated nails with mineral oil showed no increase in flexibility. But when they applied mineral oil to previously hydrated nails their flexibility was increased. This finding suggests the oil itself is not strengthening the nail against breakage, but that the oil can lock in water.
Another experiment showed that an oil and water mixture (specifically a phospholipid-water preparation for those of you keeping score at home) did boost nail flexibility. The researchers theorized that oily material that can bind and seal water in the nails are helpful in improving flexibility (and therefore, strength.)
Second, the same article also pointed out that some materials can penetrate the nail but there was no indication that penetration and strengthening were connected.
The Beauty Brains bottom line
Oil can help keep nails moisturized and keep them strong by making them more flexible so they’re less likely to chip and break. But it appears that using lotion should be better than oil alone, unless you’re hydrating your nails first and then using oil to seal in the moisture.
J. Cosmet. Sci., 50, 363-385 (November/December 1999) Structural characteristics and permeability properties of the human nail: A review GOURI V. GUPCHUP and JOEL L. ZATZ, College of Pharmacy, Rutgers University