What kind of oil or lubricant should I use for metal parts?

WD 40 is not really a lubricant. Motor oil or axle grease will provide lubrication and provide protection against corrosion so long as a continuous film covers the steel parts. One thing to remember is that grease will increase the amount of torque required to move parts )although not as much as pieces of rust). So as long as your drive produces enough torque, you should be ok. Graphite is a better lube in some situations as it does not increase the force require to move the parts but it will not provide corrosion protection. good luck