Oil is to stop moisture from initiating rust and to lubricate the mechanism on folders. If you use your knives for food the best lubricant is mineral oil ( and even some nutritionists caution it's ingestion.) Stainless steel naturally requires less ( stress on less) and carbon knives will acquire a staining or "pickling" from foods that provide a small degree of protection. WD-40 is not that good of an oil. As the first of the new "super lubricants" it was sprayed into many semi automatic firearms; which proceeded to jam from the attracted debri and associated mess as it congealed on critical lockwork. My maintenance consists of washing in VERY hot water, taking care not to damage the natural wood and bone handle materials on some. Then on my carbon blades ( usually more traditional materials and used for food) I wipe olive oil. This is the same oil I maintain the leather sheaths with. Using the same oil helps eliminate any staining from the aniline dyes and acids used in producing the leather. the only pitfall is olive oil can go rancid if applied to thickly and the leather/oil is a feast for mice improperly left lying around. For the other blades, saws etc. I tried every latest hi tech super oil as it came out. I wound up falling back on a old can of gun oil with a fancy label. I was told it's common machine oil by my gunsmith. The key word is not to much oil and just give your tool a brief look as you use it and service accordingly.