Lubricating the chain with hot 90W gear lube works but it is also efficient fly paper, collecting plenty of hardpack between sprockets and on the outside of the chain. Motor oil is far better, but motorcycle chain and chainsaw lubricants are better yet, because they have volatile solvents that allow good penetration for their relatively viscous lubricant.

That was taken from here on Sheldon Brown's site, and was written by Jobst Brandt. Motor oil won't harm your bike, and is better than nothing, but it has definite drawbacks.
I also like Phil Wood's Tenacious oil.