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It can be a daunting task choosing the right oil for your motorcycle.
Nearly all modern engine oils have two viscosities stated on the bottle. XXW-XX, where X is a number called the SAE Code or the “weight” of oil. These types of oils are called Multi-Grade Oils. This is because a modern engine needs different viscosities at different operating conditions.
Viscosity describes how well a liquid flows. An oil that is very thick, flows very slowly.Viscosity for oils is graded by how long it takes for the oil to travel through a standard orifice at standard temperatures (standard meaning strictly controlled values/environment that all tests are carried out at). The longer the oil takes to travel through the orifice, the higher its viscosity grading.
“W” Rating
The number before the “W” on an oil bottle is the viscosity of the oil at 0 degrees Celsius, the Winter rating. This gives a good indication of how an oil will behave relative to other oils at a given temperature. So on a cold morning in Auckland at around 3 degrees C, a 10W oil will be more viscous and resist flowing more than an oil with a 5W rating.
This video showing different grades of oil flowing sums up the above quite nicely.
Why is choosing the right “W” rating so important?
Your motorcycle has a lot of small orifices and galleries that the oil must travel through in order to reach your cylinder walls, bearings and other components that need lubrication from the moment the engine starts running. If the oil is too thick when cold, it will struggle to move around the engine quick enough to lubricate all the important areas inside your motor, leading to increased wear of components. A lower “W” rating will mean the oil is sufficiently thin enough at lower temperatures to make its way around your engine as required by the manufacturer of your engine. This does not mean you should choose the thinnest oil (lowest W rating) you can find! Your owners manual will state what oil grade to use based on what temperatures your motorcycle will operate in. In most cases around New Zealand, a 5W oil or 10W oil will suffice.
Hot/Operational Condition Rating
This rating is one of the most confusing and it is due to the complex science behind modern multi-grade engine oils. Although the second set of digits in the XW-XX combination of a multi-grade oil has a higher number than the W rating, this does not mean the oil is thicker when it heats up compared to its thickness when cold. The hot rating is measured at 100 degrees Celsius and is an entirely different rating given to an oil to be able to compare various oil viscosities at 100C. No matter what, as an oil heats up, it thins out and becomes runnier compared to when it was cold. But the hot viscosity rating gives us a comparison of how thick a hot oil is compared to another hot oil. So don’t go comparing the winter/cold viscosity with a hot viscosity!
As with the W rating, the higher number the hot viscosity is, the more viscous the oil is and the more it resists flowing. HOWEVER, as I said, the oil is runnier than it was when cold so it has no problem getting around the engine once hot. The most important thing when oil is warm is not its ability to get around the engine (although this is important, too). A thicker oil has a greater shear resistance, a greater film strength.
What this means is when your engine is operating under load, under intense conditions such as high RPMs, the oil must act as a barrier between the piston and cylinder walls. It must also act as a cushion between the crank and big end bearings, lubricate the camshafts and other parts of your head.
As you can imagine, the oil is under a lot of stress. If the film strength is weak – metal to metal contact can occur. This means damage can occur prematurely to your engine or in the case of sudden loss of lubrication, almost instant death of your motor such as by running big end bearings.
The loss of film strength can occur due to many scenarios but some of the most avoidable situations are:
- If you choose the wrong type of oil viscosity for your application, the oil can literally be scraped off the cylinder walls or fall out of the bearings causing metal to metal contact.
- Oil that is old and has deteriorated can struggle to reach adequate viscosity and be too thin to properly guard your engine.
- Oil that is contaminated by coolant, water or petrol can rapidly lose its lubricating properties and fail to provide adequate adhesion and film strength.
- Low oil in the system can mean not enough oil is delivered around the engine, causing rapid wear and potential engine failure.
NOW, you may be thinking that getting the thickest hot rated oil is the best way to go! But this is not necessarily a good idea either. Manufacturers specify a tolerance of oils for your motorcycle depending on operating conditions. The importance of choosing the right viscosity is demonstrated by the engine oil chart above. You may notice that a 20W-50 or 20W-40 oil is recommended for higher ambient operating temperatures and a 10W-30 oil is recommended for lower ambient temperatures.
As ambient temperatures go up, your engine also operates at a higher temperature, further thinning out the oil. Going for an oil with a higher hot weight rating means it remains viscous enough to protect your engine while still flowing around the engine as required.
If an oil is too thick for the hot operating conditions (e.g. running a 20W-50 oil at ambient temperatures below -5C in the chart above), the oil will struggle to move around the engine and get to important areas of lubrication adequately and/or be so thick it causes drag inside the engine and robs it of power whilst putting undue stress on internals such as the crankshaft due to drag.
A common thing many people aim for is to have a “high oil pressure” at all times, especially when racing in the car world. Higher oil pressures do not necessarily mean oil is flowing everywhere it needs to be, a pipe can be blocked and show a high pressure, meanwhile everything downstream of the pipe is starved of oil. Similarly, an oil which is extremely thick may show a high oil pressure on your gauge but struggle to flow around the engine.
I repeat, choosing the thickest oil possible is not always the right answer!
As you can see in the chart above, there is quite a leeway for most riders in what multi-grade oil they can use. Most of us around New Zealand, we can pick a 10W-40 all the way through to a 20W-50 oil depending on what is on special and be perfectly fine running it in the bike.
The key is to refer to your owners manual and figure out what range of oils you can run and store that away in the back of your mind until service time but if you’re ever in doubt, a good JASO MA 10W-40 will almost always see you right!
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