1) Does every motorcycle specific oil have transmission grade viscosity index improvers? More specifically Honda motorcycle mineral oil.
2) Can you shed some light on oil that is used in seperate dirt bike trannys? Oil change intervals? How long it will hold its viscocity? Can you use 20w-50 or is it better to use true 80w-85 tranny oil that is dirt bike specific?
3) I know synthetic oils are better, but what actually makes them better? Better lubrication? Holds its viscosity longer?
1) The Viscosity index measures the degree to which oil thins with heat. Thre higher the number, the less the oil thins when heated. Every mutli-grade oil has viscosity index improvers (VII's) whether they are for motorcycles or not. A 10w-40 is an oil that is as thin as a 10 wt at 70 ℉, but still as thick as SAE
40 wt at 200 ℉. This is done by starting with a 10 weight oil and adding the VII's. VII's are complex petroleum polymers that are bulky by molecular standards. They tend to curl up into a coil when cold, and uncurl or expand when hot, thickening the oil as they do. The problem comes from the fact that they are so big, and that they are relatively fragile, and fairly easily shredded by being run through a transmission, or other places where shear loads are high. As they are torn up, they can no longer keep the oil from thinning as it heats, and the viscosity drops off.
2)True transmission oils are formulated differently than gear oil. They contain different additives and are usually thicker to deal with the different kinds of lube needs a trans has. As you've noticed, there are multi-grade transmission oils, and I just told you that the problem is VII's breakdown, and that all MG oils have VII's, so what's up? The deal is that the VII's used in transmission oils are actually much tougher, and are generally up to the job. They often last for a very long time before any viscosity break down occurs. But they are also much more expensive, and since automotive engine oils don't need such things, most blenders don't use them in engine oils.
Amsoil and some other small oil blenders working with motorcycle specific oils were among the first to realize that this was a problem with bikes that shared engine oil with the trans, and much more so dirt bikes than road bikes. They started using the more expensive gearbox grade VII's in their blends about 10-15 years ago, and in the last two or three years, blenders have finally begun to really catch on to this. It's much more common now than just 3 years ago, but there's nothing on anyone's label other than Amsoil's that I know of that would tell you for sure. You have to have published lab tests or your own used oil samples to know if your oil holds up.
For some reason that no living person seems to know, SAE engine oil and SAE gear oil are graded differently. SAE 50 wt engine oil is roughly the same viscosity as SAE 90w gear oil. Here's a chart:http://www.synlube.c...ity_table_2.jpg
3)The short answer is that in general, they tolerate heat better, tolerate chemical and acid contamination better, have a higher viscosity index even before any additives are used, and because real group 4 and 5 synthetics are manufactured rather than refined, they contain no contaminants or unwanted components.
Here's a pretty good article on the subject. It's about 8 years old, and I don't agree with the author on some of the subjective points he makes, but the basic info regarding synthetic vs. petro oils, and the various kinds of synthetics is good:http://www.calsci.co...info/Oils1.html