What has not been mentioned is the fact that applying fresh oil to dirty mechanisms is pointless and can actually be harmful in that it will loosen not only old lubricants, but redistribute contaminants throughout the mechanism causing accelerated wear. Mechanisms must be cleaned, and by "cleaned" I do not mean flushed with lighter fluid, I mean disassembled and brushed with solvent then cleaned in an ultrasonic, then inspected and cleaned again if necessary; some old oils degrade to an almost shellac-like substance, this must be removed. WD-40 is a nightmare to remove once it has hardened.
In a typical basic mechanical SLR, say an old Pentax, at least two different oils and a light grease will be used. Teflon carrying lubricants can in certain areas replace grease. The amount applied is miniscule, so it pays to invest in quality lubricants.