I think a few here are confused by some of the terms used in oil jargon. The following it a cut and paste from one of my posts on another forum. Take it as you will.
Use what ever oil you want. If you follow the manufacturer's recommendations, or not is up to you. Big boy rules are in effect.
However, do yourself a favor and become INFORMED.
Basics:
Oil weight (viscosity) - The 40 in a "5w-40" weight oil is the main operating weight of a given oil and determines how well the oil will flow between bearings and tight clearances under operating conditions measured at 212° F. You want to run the lowest weight that will give you the required oil pressure at operating conditions. In all reality this has nothing to do with the protection available from a given oil, only the film viscosity - particularly the film that the bearings run on when spinning, and the film between other moving parts. A lighter weight oil can reduce the power required to drive the oil pump and other parts slightly, while decreasing windage in the engine, but at the cost of reduced life compared to a heavier weight oil of the same brand and type - but not always, some thin oils protect much better than much heavier oils. And an oil that may start out as a 5w-40 may eventually break down to a 30 weight oil in several thousand miles.
Multi-viscosity - How well the oil flows cold and hot. In multi-grade oils, the first number (5w in 5w-40) is measured at 32° F ("w-" stands for winter), and relates to how well the oil flows through the engine at start up until the engine becomes warm - as well as how well it drains back off parts and into the oil pan when cold. The difference between a 5w and a 10w is not as large as many would have you believe, and for those who never start their car below 30° F due to local weather or the car being stored in a garage, it is irrelevent other than how quickly the viscosity modifiers may break down in an oil with a wider viscosity range. The trend towards lighter and lighter multi-grade oils by manufacturers is generally an issue of mileage and emissions, not clearances and protection.
Viscosity modifiers are generally coiled, branch chain molecules that look like a spring that winds and unwinds as the oil warms and cools. In many cases, oils with wide viscosity ranges use large amounts of viscosity modifiers that shear (break down) during use - sometimes rather quickly. Some of these modifiers can build up on the sides of the pistons, rings, and on other hot parts inside the engine to the point of reducing oil flow to parts of the engine - not a good thing. This is a case where if you do not need low viscosity at low temperatures, you may be better served not to use an oil with a large viscosity range. In the summer, or if you use a pre-oiler, you will likely be better served with a 10w- or 15w-40 oil with a narrower viscosity range. Match the oil's weight to the use and needs of the engine.
On engines with variable valve technology where the oil pressure is used to control the cam timing and/or a secondary set of cam lobes, the viscosity and lubricity (and the oil's durability) can make a difference in how the engine performs and the engine's oil pressure when running. This is especially important in engines with hydraulic lifters/tappets.
Synthetic: This no longer means what it used to. A couple of years ago, Mobile (the manufacturer of most synthetic base stocks) sued Castrol for using the term synthetic on some of their oils using a mineral oil base stock with a highly modified additive package. A judge ruled in Castrol's favor, so now an oil can be called "synthetic" even if it does not have a synthetic base stock. Castrol's Syntec and Shell's Rotella synthetic are two of these "synthetic" mineral base oils - among many.
A given oil's ability to protect parts can be seen in the four ball wear test: Three ½ inch diameter steel balls are clamped together and covered with the test lubricant. A fourth ½ inch diameter steel ball is pressed into the cavity formed by the three clamped balls for three point contact, and rotated for a set duration. Lubricants are compared using the average size of the scar diameters worn on the three lower clamped balls. Not all four-ball wear tests (ASTM D4172) are the same, but this is a very good indication of how the oil protects under use. There are several specs for ASTM D4172 that can be run at several speeds from 800 rpm to 3600 rpm (or even a variable speed test) but generally the specs are - speed: 1200 rpm, load: 40kg, time: 60 min. Different speeds can result in different wear amounts.
I would mention the Falex pin and block test, other than many manufacturers do not publish this data.
Gear lubes: These are measured under completely different specs than engine oils. In fact, a 75w-90 weight gear lube is the same weight as many 10w-40 or 15w-40 engine oils. GL4 and GL5 refer to the high pressure, anti-wear properties of the oil. A GL5 lube has better high load wear protection than GL4 - sometimes this is done by increasing the amount of specific additives, but not always. Generally speaking, a gear lube has less tendency to foam, higher crush strength, and better high pressure anti-wear properties than an engine oil, however, there are several 10w-40 and 15w-40 engine oils that also have GL4 or GL5 ratings.
Oil analysis: This is a great way to check if the oil you are using meets the needs of the engine and the way you are using it. Combined with an occasional cutting open and inspecting of the oil filter's media, this is the best information you will get short of an engine tear-down and inspection.
On the subject of motorcycle specific oils in motorcycles, not all are the same, and a wet clutch is not as important as the oil's ability to offer gear protection without breaking down quickly.
Do your own research, and make your own decisions. But watch where you get your information, forums and manufacturers may not be the best sources. Please feel free to add input and corrections, I am wrong more often than I am right.
