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| Waste oil. © Eugene Brennan |
Did you know WD-40 isn't specifically a lubricant? Its primary function is as a penetrating oil, for freeing stuck bolts and similar. Waste engine oil is perfectly suitable for hedge cutter blades and garden shears., although probably not good for the environment. It's thicker than light machine oil and so stays longer in contact with metal surfaces, without being squeezed out. Lubrication is a Goldilocks process. The lubricating layer shouldn't be too thin, or surfaces will still rub together, increasing friction. If the layer is too thick, it results in viscous friction, which can cause energy loss and drag on fast moving parts in contact.