There are a few things that you need to know about selecting an engine oil for a four stroke engine. This guide is written to give an overview rather than a scientific analysis. It is also my opinion based on years working in the aircraft industry, and may be different to the opinion of others.
Multigrade oils have two numbers, as in 20W50, 10W40, etc. The first number is the thickness of the oil when it is cold, and the second is when it is hot, the higher the number the thicker the oil. Having said that cold oil at 10 will be thicker than hot oil at 40, so we need to look at the numbers individually.
When the engine is cold and we start it up, we want the oil to circulate as quickly as possible, so a smaller number is better. The life of an engine can be determined by the number of start ups, that means for the first couple of hundred revolutions, the oil has not been pumped to all lubricated surfaces, and that is when an engine wears. The oil will only stay at the cold viscosity for a few minutes, as it starts to warm it moves towards the hot viscosity (the second number). I use a 5W40 in my motorcycles for this reason.
The second number is the hot viscosity. At 95C the 40 will be thicker than the 5, but don’t forget that a 40 is more “runny” at 95C than the cold number is at 10C.
The ideal oil would be one where it is the same thickness cold as it is hot, but this would be a very expensive oil to make and would have numbers that have a large difference. Mobil make a very high quality racing oil that is 0W60.
A mineral oil is just that, oil pumped from the ground with things added to make it work in an engine. One of the things that are added is viscosity improver’s. These additives break down with use and change the viscosity of oil. That is why a car that uses mineral oil needs the oil changed more often than one used with synthetic.
Synthetic oil has other properties, one being that it is manufactured to have “long chain molecules”, in other words the oil strings are much longer than in mineral oil. To see how this works, get some hair and cut it into short 5mm lengths, put it on a table and slide your finger in it. You will find that your finger is touching the table. Now do the same with hair cut to 30mm lengths and you will find that it takes longer for your finger to touch the table – the wear surfaces spend less time touching each other. It is the case that a much thinner synthetic will lubricate better than a thick mineral.
Mineral oil will also start degrading the additives when it gets too hot (about 120C+). You oil temperature gauge will not show this, but there are places in an engine where the oil will be much hotter than where the temperature sender is. Synthetic oil will not break down till it gets to a much higher temperature, (around 150C). If you want to test this then an old frying pan, a thermometer and your cooker will show it, fried mineral will create treacle at a given temperature, whereas synthetic won’t. Most car engines with turbos on need synthetic oil. You may think that your oil never gets hot enough to need synthetic, but should you develop a slight oil leak, and that in turn reduces the amount of oil in the engine, then it will get hotter.
Semi-synthetic oils are an in between.
Some motorcycles (and some older cars) use the engine oil to lubricate the gearbox too, we should therefore look for an oil with high shear properties. Those oils are used in most Japanese motorcycles, which lubricate the gearbox in the same way. In my opinion, this part is not too important as the old style Minis also used the engine oil to lubricate the gearbox and they were happy with normal car oil.
At the end of the day you must decide on what oil you want to use with reference to the manufacturers guidelines and your own experiences. I use a fully synthetic 5W40 petrol oil in my bikes, and a fully synthetic 5W30 diesel oil in my diesel 4×4. I am quite happy to double the oil change interval with these. When you are guided by the owners handbook for your bike take into consideration the fact that oils have evolved over the years just like tyres have. If you are happy to use a 20 year old oil specification as directed by the handbook then feel free to use the 20 year old tyre types that it recommends.
At the end of the day the job of the oil is to hold two metal surfaces apart, neither the oil not the metal know if they are in a Ducati motorcycle or a 50 year old Ford.