Stay away from oil, especially WD40. Oil will cause dust and dirt to stick to the bearings and cause them to wear out just as fast, or faster, than running them dry. If your bow uses bronze bushings instead of bearings, WD40 will totally ruin them. WD40 is good for freeing rusted bolts and nuts, and removing adhesive residue, but it's worse than worthless as a lubricant.

I tear down my bow every two years and check the axles and bearings for wear, then lube the axles with white grease when I put the bow back together. If needed, I use Tetra Lube or TriFlon between overhauls.