Single or Mono Grade oil (e.g. SAE 40w)…
This is an oil which is only suitable for use within a very narrow temperature range.
SAE monograde oils usually fall into 2 main categories. The grades with a ‘w’ after them (SAE 5w, 10w, 15w and 20w) are only suitable for use in winter time because they are generally thin oils which are good for use in winter or for cold starting.
The ‘summer time’ grades (SAE 20, 30, 40 and 50) are more suited for warmer (or summer time) temperatures or for use in an already hot engine.
The problem with single grade (or mono grade) oils is that they can only operate efficiently over a small temperature range. For example, a SAE 10w oil is fairly thin which is good for cold starting but it becomes to thin for effective lubrication as the temperatures rise (as the oil gets hotter, it starts to lose it’s viscosity or thickness).
An SAE 40 oil is fairly thick, which provides good lubrication when the engine is hot, but thick oils can cause drag when used from cold (e.g. starting the motorcycle first thing in the morning).
Gear and gear box oils, which are used in engines where the gearbox is separate from the engine, are mono grade oils but they use a different range of SAE grades to measure their viscosity.
The commonest grades for gear oils are SAE 80 and SAE 90. As a very rough guide, SAE 90 gear oil has the same viscosity as SAE 50 engine oil
Gear oils used for certain applications (e.g. certain racing applications) are prefixed by the letters ‘EP’ which stands for extreme pressure
Multi Grade oil (e.g. 10w40)…
In the 1960’s, long chain polymers started to be used in engine oils to improve the oils viscosity index (basically meaning the oils were effective over a much wider temperature range).
This led to the creation of multi grade engine oils (like 10w40 or 15w50), which is the type of engine oil used by most modern motorcycles.
The most widely used multi grade oil is 10w40, which operates as a thin 10w weight oil at 0°F and as a 40 weight oil at 200°F