The old way of painting. Colors are more true and the work actually appears to have more
depth, particuliarily when working with one of the
opaques. There is a richness to be had in oil, and though it takes longer to dry than
acrylic, the product is more worth the wait.
(I carry these opinions because I was an acrylic painter for years, always trying to mimic oils, until I gave up and grabbed the linseed.)
And for the record, oil color never dries. Those paintings in museums. They're not dry. If they were entirely dry, the pigment would be dust on the floor beneath the canvas. There are a few signs, seeing as acrylic really is still a pretty new medium, that oil is by far more archival.