I'm a fan of Amsoil and a long time user of it in my other bikes and autos.
I'm interested in your take on viscosity, Reddogs.
so my take on viscosity, mostly based off of bobs the oil guy articles and general things learned through working in auto parts.
simple explination: the smaller the first # the less it thickens when cold and the better it is for your engine, the larger the 2nd and the less it thins at high temp keeping the ideal viscosity. outside temp does not matter except in extreme conditions. synthetic is better.
a little longer than i like but its hard to simplify it when theres alot to cover
first its important to know what the #s such as 5w-30 10w-30 and 10w-40 mean. ive heard w standing for winter or weight. technically the w stands for a different set of conditions while testing viscosity. outside temperature matters very little for the performance of the oil. unless you are in an area with regular freezing temperatures or desert conditions viscosity does not need to be adjusted. no matter what ambient temp your engine is designed to reach normal operating regardless of outside conditions.
now the visc scale is a little misleading the way it is numbered on oil containers. take a visc of 10(which is not 2x the viscosity of 5), this is more like a range of how a 10 weight (ill just use weight as most people say it and it makes sense even though it may not be completely accurate) will vary in viscosity as temperature changes. ill make up numbers for this part but ratios are somewhat accurate; for a 10 weight say it has a visc of 100 at room temp well that visc thins at an exponential decay rate. if you wanted to see a rough graph y=1/2^x would represent how it thins at a slowing rate. so that same 10 weight with a viscosity of 100 at room temp would have a visc of around 10 at 220 degrees and a visc of 5 at 300 degrees. a 30 weight on the other hand may have a viscosity of 200 at room temp, 15 at 220 and a visc of 6 at 300 degrees. the main thing to notice is that at low temps the grade of oil make a significant difference in viscosity but as the oil heats up to operating temp (regardless of weight) they all get closer and closer to the same viscosity. for instance a 5w-30 and a 20w-50 have very different viscosities at room temp but are nearly the same the hotter they get.
now your engine likes to run with a particular viscosity, lets call it 10 but it is impossible for any oil to be at this viscosity all the time. this is where multi grade viscosities come in i.e. 5w-30. the way i understand it is when the oil is hot it behaves like a 30 weight so around 300 degrees it has a viscosity of 6 but when it cools to room temp instead of having a viscosity of 200 it acts like a 5 weight and might have a viscosity of 80 at room temp. the purpose of this is to keep the oil closer to the ideal viscosity the engine requires.
one big concept that seems the opposite of what most people think is that you need your oil to be pretty thin, especially at startup. oil is always too thick at startup, no matter what grade you use, this is why youve probably heard most engine wear is at start up. the oil having too thick of a visc takes time to flow to engine parts causing excess wear. as the oil warms up it approaches optimum visc and more easily flows to critical engine parts.
the problem with too thick of oils: they take too long to reach optimum visc and are not able to flow into critical areas to prevent wear, also to high of visc results in higher pressures and can damage components such as collapsed filters.
so the goal of a multiviscosity is to keep the oil as thin as possible when cold to help flow on starting and to lessen the rate of thinning when warmed up so it does not get too thin at operating temp. some people may argue you want a thick oil to stick on parts so you dont get dry starts, this is inevitable as no matter what grade you use they are almost the same at operating temps( i.e. when you would turn off your engine) and the oil quickly falls off engine parts as soon as flow stops, again no matter what grade you use. think about it, have you ever rode home waited a couple minutes and checked your valves? in the 15 min it takes to get to your valves they have almost no oil on them. there will always be a small point of time when you restart where the residual oil is removed and metal on metal contact occurs before oil flow is restored to lubricate. an oil that is closer to ideal viscosity(usually thinner at cold temp) it able to flow faster to parts and reduce the time metal on metal contact occurs.
for people who say if they use a thin oil it will leak when sitting, oil is much thicker when at room temp, if anything your engine would leak much worse at running temp and you should fix the leak. that is not to say thicker does not help with leaks, but thicker also leads to poorer lubrication to mechanical parts.
i take my duke as example, i use a 15w-50 synth. i have collapsed several filters and i burn some oil when im pinned for extended periods of time. the reasoning: my oil is slightly too thick the first few minutes of riding resulting in excess pressure on the filter and when im pinned(extreme high temps) my oil is a little to thin resulting in burning. i think i would benefit from a 10w-60 instead of a15w-50 and what do you know this is the recomended visc for the duke.
synthetics briefly: synthetic follow the same principle except the viscosity change is not as great, they are closer to ideal viscosity all the time compared to conventional. not to mention they are better lubricants, last longer and blah blah blah. there is no downside to using synth on old or high mileage vehicles, you can switch back and fourth between synth and conventional altho they should not be mixed. SYNTHETIC IS ALWAYS BETTER
the main goal when looking for oil, synthetic is better period. general rule of thumb the thinner the oil is at startup(the lower the first #) and the thicker it is at extreme temps(the higher the second #) the better it is. this is just a guide tho some engines may not like an oil that is thicker at extreme temps(i.e. a bike that is not run very hard and may not get up to full operating temps). most of the time a thinner oil at start up is an advantage tho but you can go too thin to where it thins too much at normal operating temp and will probably burn some oil.
more explinations can be found on bobs the oil guy and im always open to others opinion of what works good and what doesnt work good and why