Originally Posted by Svr

I've always had great luck with Finish Line Cross Country lube (green cap). It's an oil, and it's possable for the chain to gunk up if it's over applied. The key is to apply one drop per link, rotate the chain through a few revolutions, then wipe the exterior of the chain with a rag. Lube does nothing on the outside of the chain but collect dirt. Very little is required per link for good results.

You'll still see some gunk buildup after several rides, but a quick wipedown will keep things reasonably clean.

My guess is that the shop guy doesn't want to deal with an oily/dirty/nasty drivetrain when/if the bike is brought in for service. Many people use the "if a little is good, then more is better" way of thinking when oiling a chain and end up with a terrible mess.